Steam lets each account play up to 32 games at the same time. We rotate through the games you pick so every one of them earns real playtime, not just the first on the list. Trading-card drops spread across the whole list too, so you're never stuck on a game that's already given you all its cards.
How many games you can load per account depends on your plan: the Free plan covers 10 games each, Starter 15, and Pro and Lifetime go all the way to Steam's maximum of 32. Every plan still rotates through whatever list you set.