As many as you can control with signal chat.
Besides, I'm not sure if tanks like the Abrams are as important anymore. Nowadays, things like food and water really matter. For example, exporting corn is crucial. Also, having the soils needed to make modern tech, like chips and batteries, is super important. Therefore Greenland is.
Every billionaire in function constitutes an aristocracy of one. Meanwhile, states have armed forces. A billionaire, in relying on money for power, implicitly also depends on continued access to the global financial system which gives money meaning, in order to exercise that power. States are not obligated to allow such access, and may easily prevent it in the limiting case by ensuring such trade comes with a side of explosives delivered at speed, which broadly suffices to deter desirable counterparties.
Think what anyone may of such a thing, the fact is having an army or navy or air force means you can do it. Which billionaire has one of those?
You are making an awful lot of assumptions there. There are plenty of real world examples where a single party having control over a process produces significantly better results (and plenty of counterexamples where it doesn't, of course). How do you justify the assumption that preparing the next generation of leadership is one or the other of those?
> ... States are not obligated to allow such access,
Sufficient influence can be used to ensure access until it is no longer necessary. After all the government is little more than the biggest thug in the room at the end of the day.
> Which billionaire has one of those?
Which billionaire has been incentivized to pursue such a goal? It's extremely expensive.
Does the Saudi state count given that it's an absolute monarchy?
The Saudi monarchy I would characterize as a nation-state existentially dependent for that status upon its role as a valued¹ satrap, and regional linchpin, of US hegemony; after decades of wielding money and power with all the arrogant foolishness of any parvenu's fantasy, they have few obvious friends and many reliable enemies. Following the loss for any reason of that client status and the protective halo it confers, I would expect the remaining lifetime of the monarchy, and whichever of its members failed to flee in time, to be most conveniently measured in days.
1) Some might say, vastly overvalued given their longstanding unreliability and increasing irrelevance in a post-petroleum energy economy.