How many people were knew about the atomic bomb?

Answer 1

Very few people knew ANYTHING about the atomic bomb. First of all it was customary to keep the departments in the dark about the whole picture. In Los Alamos, New Mexico was a compound where the research went on. When the two bombs were ready they were totally different and secretly transported under tarps so not even the handlers could see what they looked like.
Watch the documentary "Fat Man and Little Boy" to get the sense of how things were.

Answer 2

None. The first atomic bomb was detonated in a controlled area, with all personnel in secure places.

The first atomic bombing is believed to have killed 70 to 80 thousand people immediately; this was followed by a further 10 to 70 thousand by the end of 1945, and another 60 to 110 thousand by 1950. Negative effects continue today; many people have died in the last two decades via aftereffects of the blast.

Answer 3

90,000-166,000 people in Hiroshima and 60,000-80,000 in Nagasaki.