I don't think we have a number for "how many people." That's the kind of data that is hard to get in any event, certainly after something like the bombing of Nagasaki.

But the fact that Truman had announced Hiroshima had been hit by an atomic bomb was broadcast on Japanese radio on August 6th, and the some real reporting on Hiroshima was available on August 8th, but the most detailed descriptions of Hiroshima, including the casualties, were not available until August 9th, at which point they wouldn't have been of much use to anyone in Nagasaki.

There were also some Hiroshima survivors who went to Nagasaki after the Hiroshima, and were "double-bombed."

So it's entirely possible that people in Nagasaki would have been able to have some idea of what had happened to Hiroshima prior to their attack. There was no way for them to have known that they would have been the second attack, or when it might be coming — indeed, Nagasaki was not even meant to be the second target (Kokura was, but clouds/smoke made them go to a secondary target), and did not quite have the same "unbombed" status that Hiroshima had (it had been conventionally bombed several times during the war, most recently as late as August 1, 1945).