but "to those who think we should've dropped one bomb as a demonstration."

From all I've read, we had enough nuclear material for 3-4 bombs by the summer of 1945. One was tested in New Mexico, one was dropped on Hiroshima, and another was dropped on Nagasaki. I don't know for sure if we had sufficient material to make any more bombs but even if we did, it probably was only enough for one more. There certainly weren't enough bombs to risk wasting one on a demonstration. That was a good decision, IMO. After all, what possible demonstration would've been more effective than actually dropping a bomb on a city?

Here's a historical tidbit: the nuclear physicists in New Mexico invented two different types of A bomb. One type, the "Little Boy" design, used two subcritical pieces of U235 that were brought together in a sort of cannon. The other design was the "Fat Man" type that used the implosion of plutionium to start the reaction. The bomb tested in New Mexico was of the Fat Man type. They were so confident of the Little Boy design that they never bothered to test it before dropping one on Hiroshima. The bomb used on Nagasaki was the second Fat Man device.

According to http://www.fas.org/nuke/hew/Usa/Med/Lbfm.html , it would've taken several months to make another Little Boy device. They had several Fat Man bomb assemblies ready to go minus the plutonium. I'm not certain about how much plutonium was available at the time or how long it would've taken to make more.

As an aside, Nagasaki was a secondary target on Aug 9, 1945. The primary target was Kokura but the weather there was too bad so Nagasaki was bombed instead. I can't help but wonder what the people of Kokura think about how close they came to being bombed.