Actually, there weren’t. If I recall, they only had one morwe ready to go.
Here’s what Gen. Groves had to say about a third bomb.
http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB162/67.pdf
I believe we would have found a way to fabricate several more bombs purty damn quick if we had needed them. I would be interested to see any research on how many bombs were built immediately after hostilities ceased and when they were ready to deliver.