How many times should the followers say 'Ameen' behind the Imam when he's reciting Du'a al-Qunoot?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. There is no given number of Taamin (saying Ameen) as it is to be said any time the Imam makes a Du'a. But the Maamum (person following an Imam in a given prayer) is not supposed to say Ameen after the Imam praises Allah. Allah knows best.