if the dowry has been agreed between the wali and the male suitor, do the two witnesses at the time of nikkah need to be made aware of how much the dowry is in order for the marriage contract to be valid?

Ustadh AbdulAziz al-Haqqan

No, the mahr is not a condition for the validity of the contract, so the witnesses do not need to know its amount. What they witness is the offer and the acceptance.