There's actually quite a bit in our FAQ about the atomic bombs, bomb targeting, and what actually prompted the Japanese to surrender (the bombs, the Soviet invasion of Manchuria, etc.) You can find that section of the FAQ here.

To perhaps give you a short answer, there were two atomic bombs used on Japan because that's how many the Army had on hand before Truman halted further bombings (contrary to popular opinion, Truman never made a positive decision to use the atomic bombs; he did make a positive decision to stop using them without his personal permission).

The reason why the Army had two on hand and more in the pipeline is related to how quickly the bombs could be constructed, which was related to how quickly plutonium could be produced, but there was never a plan to drop just one bomb on Japan and see what happened. This is the "traditional" narrative that came out after the war -- the bombs were used to prevent a land invasion -- but it's a postwar narrative and not accurate.

The military's plan was always a both/and, not an either/or -- the conventional firebombing of Japan would continue, atomic bombings would continue, invasion plans would continue, the blockade of Japan would continue, the Soviets would invade.

For two specific posts on this, see:

https://www.reddit.com/r/AskHistorians/comments/2todt6/did_the_us_have_to_nuke_japan_in_wwii/co17rtk

and

https://www.reddit.com/r/AskHistorians/comments/c34bcf/did_the_atomic_bombings_in_japan_indirectly/