Be prepared for them to grow, and to grow fast. I am not able to recomend you with a good number, 8 seems good to me, maybe even 10. Just remember, they can get up to 3 inches, my two oldest ones are not even 1.5 years yet and they are nearly 3 inches, well just over 2.5.
I feed them a bit of everything, they are not picky eaters and will eat whatever is given to them.
I feed them a lot of frozen brine shrimp, shrimp pellets, spinage, cuccumber, peas, lettuce, live food whenever I can get it, algae wafers, they will even eat very small pellets. Like I said, I feed them almost anythign and everything If they even think it might be food they will try and eat it.