Without Serious Handwaving, Zero
Such a world is not structurally stable. The theoretical largest possible structure of this nature using carbon nanotubes (10,000 km) is shorter than the diameter of the earth (~12,750 km). A spire of your proposed length would collapse under its own gravity into a molten sphere of carbon nanotubes in a horrifying catastrophe not unlike a mole of moles. I find it extremely implausible that even hyper-advanced material science is able to produce a material over 300x stronger than carbon fiber that can somehow resist the cosmic tendency for very large things to collapse into spheres. Gravity always wins.
This structure is long enough that it will basically behave like a giant uncooked noodle (maybe even a cooked noodle, depending on the material's ability to bend). A large enough imbalance of mass, particularly at either end, would send this structure careening out of control (I assume that gravity is simulated via rotation) and cause it to shatter into pieces or, at the very least, cause frequent severe earthquakes throughout most of the structure.
If somehow physically possible, there are still other problems.
There's also the issue of gravitational pull toward the center of the spire. The extreme ends of the spire will feel like steep mountains and potentially not be inhabitable as a result. The spire needs to apply antigravity at the extremes for this setting to work.
Transporting goods and data is a huge logistic problem. A message sent from one end of the spire to the other via light will take approximately 6 seconds (1,000,000 miles / 186,000 miles/second) to arrive. This doesn't take into account signal loss. Keep in mind that this distance is a little bit more than 2 round trips to the moon.
If you had some sort of magnetic transport system like the Bullet Train, you could achieve speeds of at least 125 MPH, probably a lot more by avoiding air resistance, but even a gain of a factor of 10 is going to come at massively higher energy costs and thermal output. If you somehow achieved a practical 100x speed gain (for 12,500 MPH), it would still take 80 hours to transport a good from one end of the spire to the other. Realistically, you're going to be stuck with much lower speeds for everyday use, effectively isolating the extreme ends of the spire from each other.
Spire units will need to be fairly self-sufficient and can only realistically trade within a dozen units or so. If each unit has its own sovereign government, chances are high there will be progressive tariffs at each unit along the way, limiting trade even further.
As far as population goes, 1 million * pi hundred = pi hundred million square miles. At 10 people per square mile on average, that gives you a total population of about pi billion people. Higher population densities are not out of the question, putting a theoretical limit somewhere around 30 billion people, possibly more.
You wanted science-based, so I'm giving you hard reality. I suggest removing that tag if you intend to pursue this setting seriously.