You could probably fit quite the few of them, I wouldnt do no more then 1 male to 2 maybe 3 females.

You ask what to feed them? This makes me concerned did you take them from the wild? If so they may be hard to feed and have some very specific requierments that differ from common kept newts.

Feeding also depends on age, of course adults can take on small fish like guppy fry but should be fed blood worms either frozen or live, black worms and so on. Efts are harder to feed and require very small foods but blood worms might be ok if you thaw the cube and slice them in small bits.

As long as you didnt steal them from nature I say good luck! and make sure you give the proper housing acording to their stage of life.