Where is the number of Bitcoins limit specified? Reading the original Nakamoto paper, I cannot see any discussion or reference to the upper bound on the number of bitcoins.

Where is it specified that at most 21 million bitcoins will be minted?
Every 210000 blocks, the reward for mining that block halves. It started out at 50BTC and now it's at 25BTC. Eventually it'll get to zero and no more bitcoins will be made.