How many times can I download Minecraft after I pay for it once?

Dude you can download minecraft at ANY computer. You should just go to minecraft.net, login your account and go install it!
note: it should be your account

As many times as you like. When you sign into the game, they know it's you, and that you're paid up, so long as you aren't trying to play in multiple places at the same time using the same login.