There are several ways to parse this question. If you're asking, how many knew an atomic bomb was going to be dropped on Japan in early August — the answer is a fair number would have been able to guess it (e.g. the thousand or so scientists who knew about the successful test in mid-July), a not inconsiderable number knew it for sure (e.g. the military people involved with the actual operations, on the order of a several dozen people).

If you're asking about the number who knew August 6th would be the date — it's a dozen or so (the military planners and crew, and a couple people at the highest levels of the project who were kept abreast on the specific date of use).

As for other world leaders — are you talking about reacting to knowledge that the attacks were pending? The only other leaders who knew that explicitly were the heads of Canada and the UK, because they were kept in the loop under the Quebec Agreement. They had to explicitly sign off on the idea. The Soviets knew the US had a bomb but did not know exactly when it would be used.

If you are asking about world leader reactions to the bombings — that is a much longer response and arguably hard to do (because "world leaders" is a pretty big category).