my boyfriend drinks 4 a day on gym days, but he only uses milk for one of them. I'm not sure how many scoops he has a day though because I know for at least the morning and after the gym he uses 2. He has done a lot of research into his diet and work out routine though, so I wouldnt drink that many without doing some serious research of your own.
I drink 1 a day in the morning on gym days, and occasionally I'll have one as a before bed snack.