In 1945 the US only had 2 A-bombs ready for use. The fat man and the little boy. And they used them both. But the only other information I could find was that the US had factories at the ready to make more. This probably would have forced the US to pause a bit whether they wanted to or not.
If they had say, 25 bombs at the ready instead of 2, how many would they have used? What is the mindset of the military leaders in 1945? Would Groves and Truman have waited to see what happened after the first 2? It appears Groves was very eager to use the bombs he had. Was the damage being witnessed making anyone in the us military leaders think twice? Or would the US have bombed much more if they could have?
Here's an answer I gave to the basic production question awhile back.
As for how many they would have used had the situation been different — I don't think we have any real idea. As it was, the first bomb was more or less explicitly approved by Truman; the second bomb was more or less a surprise (the decision to drop it was made by people local to Tinian); the third bomb was explicitly halted by Truman. That seems to indicate that they would not have kept dropping an endless stream of them, but who knows. Truman's attitude towards the atomic bomb (as measured in his language about it) changed very dramatically after August 8th, when he got the first damage and casualty reports.