Is it Sunnah to say "Aameen" after each and every Du'aa made by the Imaam, or just to say it once, at the end of all the Du'aas? Please advise, with supporting Hadeeth, if possible.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

As regards your question about saying 'Aameen' behind the Imaam, the Sunnah in this regard is to say 'Aameen' after every supplication rather than once at the conclusion of the supplications. The evidence for this ruling is the report that the Prophet regarding a Muslim who makes a supplication for his Muslim brother while absent said: "There is an appointed angel at his head, every time that he makes a supplication for his brother in his absence the appointed angel would say thereupon: "Aameen, and for you, the same thing." [Muslim]

Allaah knows best.