To answer your actual question though, we need about 85 times our current pumped hydro capacity to transition to a fully renewable US. This seems daunting, but:

• Pumped hydro is growing rapidly
• It's not the only battery storage technology (heat batteries look promising imo)
• Any increases in storage allow more renewables, less pollution, and overall contribute to making our future better

Pumped Hydro doesn't need to singlehandedly handle the storage load of the entire US because there are other options to use in conjunction with it and even a partial storage solution produces benefits. This is good, because Pumped Hydro is geographically limited.