Warfare is complex, and often not what you think
It is more complex than simple swords and shields. Multi-faceted political, tactical, strategic, intelligence, procurement, cultural, and logistical factors all influence outcomes.
In your world, magic would be a 'subset' of all these factors, not necessarily a determinant. Ie. just like the advent of gunpowder, or missiles, it alters the factors, but the factors do not diminish.
- At what level of technology, will magic become redundant?
Magic would never be redundant. Just like hand-to-hand combat is still trained for elite troops, or marching is still a major part of a military, or knives are still a part of a standard kit - Magic would be like any other aspect of the military that would exist even if it is weak or simple. It would be another tool to use in a kit, or a cultural factor to be used in a military.
- If technology is at around 17th century, then how powerful a magician needs to be in order to destroy entire army alone?
An entire army can be destroyed by the weather (a simple winter). Or a bad commander, who moves his troops too far away from water. I would put forth a magician need not be powerful at all - aspects of deception and good planning can easily outwit an army especially a poorly organised one, or one easily deceived. Your magician can simply feed misinformation, or lure the army to its doom. Worst has happened to armies even without a magician.
- Capturing cities and maintaining law and order in conquered places need lot of troops. Can powerful magic circumvent this need?
Of course - although it is not needed. A good strategy could be to even hold a threat in-being. Like a fleet-in-being, you could simply be too powerful to resist, and you don't need to commit troops at all. The Mongols intentionally kicked up dust to look like their numbers were huge - cities surrendered to them without fighting because they thought it was better than fighting.
- Does it make sense to raise armies if a single high leveled magician can waste it alone using hit and run tactics?
Again, warfare is complex. A good commander up against a seemingly powerful magician uses his strength against the magicians weakness. An army could be needed, or simply a small group. Where can you position your forces to capture him? How can you deceive him? Can you chase him to his doom, or lure him to it too? How much does he know, and how mentally tough is he? Can you grind him down through attrition, or outwit him through deception? Are you more disciplined than he?
You may notice none of these questions have anything to do with magic - it does not matter if it's a 17th century group against a modern tank, or a magician against a soldier. In the end aspects of the facets of warfare come into play independent of the level of technology, or the level of magic.