[ADLinden]

I don't think it really matters how much you are playing at a certain time, just play as man y as you can play without decreasing your play. Usually people play with 100ish buyins in their roll. But for serious pros it can be more, or for others it can be lower if you are willing to move down when you struggle if need be.
With a roll of 500 I would be playing the 6.50s though, you dont wanna mess around at the 3's and their rake. I would actually play the 6.50s with amounts much smaller than that fwiw. ( I started with a br around 200 when i got into Sngs. fwiw)