Hello there,

In order to accurately give you a good answer on how many we need to know the size of your tank. Is that a 5 or 10 gallon?
If its a 5 gallon you can get 10 babies in there.. If its a 10 you can get 15-20. NO MORE!
You can let them go immediately as soon as their tails are absorbed.
Also I would HIGHLY recommend you get a tank that is suitable for fruit-flies. Im sorry you need to think of the needs of the frog first. They will not start eating until their tail is completely absorbed. They will eat fruit-flies for their first week or two. After that PINHEAD crickets for the next couple weeks.
I would also put springtails in there as well. If you want to avoid the fruit fly route. But in all reality a variety is what those frogs needs as well as any other. Good luck =)