Polymorphous Rush targeting Young Pyromancer and X Tokens
Young Pyromancer triggers when you cast a spell. Polymorphous Rush is just one spell, no matter how many targets you give it, so it triggers your Pyromancer just once. Once it resolves, you'll have plenty of Pyromancers around, so if you cast another spell you can trigger all of them.

Remember, you cast a spell, it goes on the stack, and then it resolves. (After anything else that happened in response, of course.) Young Pyromancer triggers when it's cast, and the tokens are turned into copies when it resolves.