I have a macbook pro and i am looking to get a gaming pc. Ive downloaded a few games on steam that are mac and windows compatible. I play them on my mac. Would i be able to sign into my steam account on my windows pc and download the windows version without having to buy it again?
You are allowed to activate your account and download/install your games on as many machines as you want. But the usage of a steam account is restricted to only one machine a time.
You are not allowed to play games on multiple machines simultaneously! This restriction is also technically enforced. As noted by Nolonar in the comments you can be lgged in on multiple machines at the same time.