If there is one thing I can't stand about real or fictional situations, it is illogic.
In the Star Trek: Deep Space Nine episode "Sacrifice of Angels" a Federation and allied fleet of 600 space warships fought a Dominion Fleet of 1,200 space warships in an important battle in the war. And you might think that 600 ships vs 1,200 is a very impressive space battle.
Our Milky Way Galaxy has a galactic disc about 100,000 light years in diameter and about 1,000 light years thick. Suppose that the United Federation of Planets occupies a cylinder 1,000 light years "high" and with a radius of 500 light years & diameter of 1,000 light years. Such a cylinder would have a volume of 785,398,163.40 cubic light years. With 0.004 stars per cubic light year, there would be about 3,141,592.654 stars in that volume.
If one star out of 10, or 100, or 1,000 in that volume has an advanced industrialized planet which joined the Federation, there would be 314,159, or 31,415, or 3,141 advanced Federation member planets. If each such planet could build ten space battleships per year, and train crews for them, the Federation could add, 3,141,592, or 314,159, or 31,415 space battleships to its fleet every year.
If the Federation was 1,000 light years in diameter, it would appear as a tiny circle one per cent of the diameter of the galactic disc in a map showing the entire galactic disc. Of course the space maps seen in various Star Trek productions show the Federation and the Dominion several times that large, implying that they should have even larger fleets of warships than calculated above.
But suppose that the Federation and the Dominion could find only 600 and 1,200 ships for this important battle in the war. The Federation fleet with 600 ships would still have more ships than the British and German fleets at the Battle of Jutland combined. And if you look up the organization of the two battle fleets at Jutland, each had several admirals commanding various units of ships in the battle.
But in "Sacrifice of Angels" the 600 ships are commanded by Benjamin Sisko - captain Bejamin Sisko!
I found that highly illogical and hard to believe.
When I was a child I read a book, probably a Disney book, with a lot of stories, and one was the mostly true story of Old Abe the war eagle (1861-1881), the bald eagle mascot of the Eighth Wisconsin Volunteers in the US Civil War.
I remember that the story briefly described the organization of the Union Army. About a hundred men were in an infantry company and each infantry regiment had ten companies and so about a thousand men when it was mustered in - but not when it was mustered out.
Several regiments made up a brigade, and several brigades made up a division, and several divisions made up an army corps, and several corps made up a field army. And I was really impressed by the thought of such vast numbers of men, wondering where they could find enough soldiers for the Rebel army with so many men in one Union field army.
And it is a good idea for writers to sometimes let themselves be impressed by the vast numbers that may be involved in their stories.
Decades later, I learned about the general ranks in the Union army. There were only two, brigadier general and major general. Except that Winfield Scott, the commanding general of the United Stars Army from 1841 to November 1, 1861, was a major general from 1841 and a brevet lieutenant general from March 29, 1847. A brevet rank was a sort of an honorary rank, to greatly oversimplify. And Ulysses S. Grant, the commanding general from March 9, 1894 to March 6, 1869, had the full ranks of Lieutenant general from March 4, 1864 and general from July 25 1866.
So from November I, 1861 to March 4, 1864, the commanding general of the US Army had the rank of major general.
Thus brigades were commanded by colonels or brigadier generals, divisions were commanded by brigadier generals or major generals, corps were commanded by major generals, field armies were commanded by major generals, and the entire Union army was commanded by a major general more often than by a lieutenant general.
The Union army also had territorial commands. A number of districts were often grouped to form a department (which often had a field army attached) and a number of departments were grouped to form a military division. There were times during the war when a general had several field armies under their command while being their self under the command of the commanding general of the Union Army.
For example, for several months Major General Henry W. Halleck had several field armies under his command while being under the command of Major General George McClellan, the commander of the Union army. At that time officers with the rank of major general commanded five separate levels from divisions to the entire Union army.
I consider that highly illogical.
And it seems to me that if an army has five levels of command above a brigadier general's level of command, the officers at the top level should be five grades higher than a brigadier general.
In the 20th century military units were often much larger than during the US Civil War.
A division is a large military unit or formation, usually consisting of between 6,000 and 25,000 soldiers.
The size of a corps varies greatly, but two to five divisions and anywhere from 40,000 to 80,000 are the numbers stated by the US Department of Defense.
A field army (or numbered army or simply army) is a military formation in many armed forces, composed of two or more corps. It may be subordinate to an army group. Air armies are the equivalent formations in air forces, and fleets in navies. A field army is composed of 80,000 to 300,000 soldiers.
An army group is a military organization consisting of several field armies, which is self-sufficient for indefinite periods. It is usually responsible for a particular geographic area. An army group is the largest field organization handled by a single commander – usually a full general or field marshal – and it generally includes between 400,000 and 1,000,000 soldiers.
The question says:
I am wondering what the officer ranks would look like for an organization with tens of millions, if not hundreds of millions, of people and/or droids. Would six-star ranks or above exist, and what would they be called? What would their units be called?
So if you are imagining an army with between ten million (10,000,000) )and one billion (1,000,000,000) human and/or robot soldiers, then you may need several more levels of command officers.
An army of ten million might be ten times as large as an army group of one million soldiers and might be content with a five star equivalent rank in command of army groups reporting directly to a six star equivalent rank commanding the entire army. Or it might possibly have one or two intermediate organizations, bumping the commander of the entire army up to a seven or eight star general equivalent.
An army of one billion might be 2,500 times as numerous as an army group of 400,000 thousand men, and so it might have space for many intermediate levels of command and many ranks of generals.
for example, if army groups have 400,000 soldiers, and if all higher organizations contain two of the lower level of organization, the level above an army group will, have 800,000 soldiers, the next level above that will have 1,600,000, the next level 3,200,000, the next ne level 6,400,000, the next level 12,800,000, the next level will have 25,600,000, the next level 51,200,000 and so on.
But if levels above army groups are organized in groups of ten smaller units, the level about an army group will have 4,000,000 soldiers, the next higher level will have 40,000,000, the next higher level will have 400,000,000 soldiers, and two and a half of them will will be beneath the commander of the entire army. So the commander of the entire army would be equivalent to "merely" a nine star general.
The US army has four general ranks, brigadier general, major general, lieutenant general, and general. Since many World War II allies had a rank above general, field marshal, the rank of five star general (and admiral) was created for the very top US officers in World War II and hasn't been created again. The rank was General of the Army or Admiral of the Navy.
Thee is some uncertainty whether the special ranks created for General Pershing, Admiral Dewey, and the special posthumous rank of George Washington, should be counted as four star ranks, five star ranks, or six star ranks.
So possibly you might want to use field marshal as the equivalent of a five star general and commander of an army group.
Then you might want to create a rank of second level field Marshall for the next higher position, and third order field marshal for the position above that, and so on.
Or you might want to revive the medieval rank of constable, which was above a medieval marshal, as the rank above a field marshal and equivalent to a six star general. And maybe a constable to the second power could be equivalent to a seven star general, a constable to the third power could be equivalent to an eight star general, and so on.
Or maybe you could call an army group a squad of armies, commanded by a second lieutenant of generals, and the group above a platoon of armies commanded by a lieutenant of generals, and the group above a company of armies commanded by a captain of generals, and the group above a battalion of armies commanded by a lieutenant colonel of generals, and the group above a regiment of armies, commanded by a colonel of armies, and the group above a brigade of armies commanded by a brigadier general of generals, and the group above a division of armies commanded by a major general of generals, and the group above a corps of armies commanded by a lieutenant general of of generals, and the group above a an army of of armies, commanded by a general of generals.
If your vast army of at least tens of millions of soldiers is designed to conquer a lot of territory and people, it will need special units to govern conquered territory. And they should not be organized like fighting units, but should have a different type of organization suited to policing conquered territories and people. And if you have many millions of soldiers as such military police in occupied and/or conquered territory, they will have many levels of units and many levels of ranks.
I hope that I have given some good suggestions about the names of very large military units and the ranks of officers commanding them.