At some point you get limited by fill/discharge rate, but the cost of storage in a big pumped hydro is still pretty cheap.
Lake Travis already has a power plant and is rarely every full for example. No one is going to start using pumped hydro there because there is no extra water to pump.
You don't actually need an excess of water, you need to be able to move enough between lakes though. You are not using any extra water from the system by adding pumps.
That's the real determining factor right, places with cheap big pumped hydro projects ideally already have big dams that are not full, so from that point it works. But then you need a reasonable amount of elevation between reasonably close lakes (or ideal new spots).