Had Japan not surrendered, we had another nuke en route (The demon core was around already, we just had to slap it into the mechanism), and could have had a fourth reasonably quickly. (We had processed a huge chunk of the fuel for the fourth pit before we dropped our first bomb) It was just after that where the US couldn’t just keep setting them off because the timescales to generate the plutonium was measured in several weeks.

Last Podcast on the Left did a decent, though immensely gory, series on the Manhattan Project recently.