There was a third bomb, ready for deployment on August 19. Relevant authorities were reluctant to use it, although there were plans for serial atomic bombings if Japan didn’t surrender. And there were elements in the Japanese Army command structure adamantly opposed to surrender.
The Emperor saw things differently and forced the army to surrender.
The fissionable material enabled the US to build twelve operational bombs (IOW, not counting the test shot in NM). Bomb construction was only swift enough to deliver one about every few weeks, but the nuking could have gone on until December, probably with the last one destroying Tokyo.
I recall reading that the ‘idiot proof’ electrical connection for the detonation sequence was seen to have been installed backwards. A couple of the scientists who’d helped design bomb accompanied it to the Far East, and they didn’t wait for orders. Somewhere in theater, before it got loaded, they got out the soldering iron and fixed it.
Otherwise, the inert weapon would have dropped through a roof and killed one guy, and the left would still be bitching about that.