I was wondering, how much HP naturally aspirated or forced induction motors can have and still get away with running on pump gas? Generally speaking, at what point would you have to start looking at other options?

You read about people talking about certain motor builds and "running on pump gas". So what is the ceiling of HP to run on pump gas?

I would guess in the 800HP range, you know what I really don't know. I have seen 800HP Corvettes running on pump gas. There might be higher HP on pump gas.

I'm at 680 rwhp on pump gas and SC.

does meth count?

There is no finite limit... You could build a motor large enough and do it in such a way to break 1000HP on pump gas, by which point you would have something running "street" fuel that is completely unuseable on the street.
The real issue though is that it becomes more and more expensive to do that (you have a 10 thousand dollar motor making the power a stock engine could make on race fuel), and more risky also... Detonation will destroy ANY motor, no matter how well built it is, and shooting for record power numbers on low octane fuel is just begging for det...

Now, for a straightforward answer... You can hit 700+ WHP on pump with a low compression forged stroker running boost, more with methanol injection.
Naturally Aspirated I think 600ish WHP would be the limit, but if you found a way to put a seriously huge motor in your car you could get more.