On all magic cards from "Exodus" (circa 1999) to present, there is a small symbol at the bottom right of the illustration that signifies what set a magic card is from. The trick is being able to put a name to each different symbol.
If you mean the standard Block Sets, that would be the Zendikar Block.Zendikar, WorldWake and Rise of the Eldrazi.M11 is due to be released July 16 2010.
there is over 15,000 cards atleast if you collected one of each from alpha to the newest set and more are created all the time, so to get a dead on number would almost be impossible. just count them...