What is the upper limit for a charge-pump voltage booster?
When stacked indeed it can go well into the kV range using a charge-pump voltage booster. In fact it is a very common approach.
Here is an image of such a high-voltage setup.

The proper name for this circuit is a &quot;Cockcroft–Walton generator&quot;. Here is the wikipedia on that circuit:
https://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_generator
Here is a video of a 75KV version:
https://youtu.be/3_rdzmfdlpo
There have been designs that go even much higher than that. Here is a picture of a 3MV (3000KV) version:

As for the maximum voltage you can achieve, there is no real limit in theory other than arcing, which is one reason these designs are useful for extremely high voltages. Each capacitor in the stack is exposed to the same relative voltage across it since it is only connected between each stage, as are the diodes. so you can stack as many as you like without any real limit so long as you keep the high voltage leads far enough away from anything it could arc to.
However in practice there is a practical limit. As the voltage increases even if this is sitting out in the middle of free space the increased voltage is going to mean increased leakage, and that means there is going to be some current flowing. The higher the voltage the more current flows. So at a certain point the thing will leak and the capacitors wont fully charge anymore. so you'd have to use bigger and bigger components until it becomes impractical.
This problem is exaggerated by the fact that the more stages you have in the circuit the higher the impedance (increasing exponentially) and the less current it can deliver before the voltage will start to drop. In practice even with very few stages it can't deliver very much current at all. This is why even a small leakage current can quickly limit the circuits effectiveness.