Exactly 21 millions, take a look at the bitcoin wiki, you will find some interesting informations:
http://it.wikipedia.org/wiki/Bitcoin
http://it.wikipedia.org/wiki/Bitcoin
keep in mind, however, that this hard limit can always be changed by altering the source code, should devs decide to do that. in the case of such a fork, it would be up to the miners to prevent the new fork from developing, and to keep the old code intact.
highly unlikely, but just throwing it out there.
Devs could do whatever they want. It is up to the miners and the network though, whether those changes really become reality! That's the beauty of Bitcoin everyone can decide what he agrees to or just leave it be.