If they do actually have a $500M valuation, then the only way for you to make money off of the stocks is for them to be bought for several billion dollars or go public for several billion dollars. The valuations are typically set at investment rounds with the investors, and investors get first preference to get all of the money back plus however much they forced the startup to agree to on top of the money they put in. (Example, 2x preference is common and means the investors would get double the money they put in before the founders or any employees get anything.)
So the startup needs to grow their valuation tremendously in order for employees to get anything. You'll have to decide whether that is likely or not. If they have that many people after series a, either they have amazing traction in their market and can grow very fast from revenue, or their burn rate is huge and they're hiring way too fast without actually selling anything. Figure out what the revenue is right now and you'll know which one and whether they're going to survive.