The original Magic core set was Limited Edition Alpha, which contained 285 cards, not counting basic lands. Then all of those cards were reprinted in Limited Edition Beta, which also added Circle of Protection: Black and Volcanic Island, which had been mysteriously left out of Alpha.
Then Revised had a slightly different set of cards, and only 250 of the cards from Alpha were still in the core set. Fourth Edition changed the core set again, and only 211 cards had been in every core set.
You get the idea. Here's the complete breakdown:
Alpha : 285
Fourth Edition: 211
Fifth Edition: 151
Sixth Edition: 79
Seventh Edition: 60
Eighth Edition: 40
Ninth Edition: 24
Tenth Edition: 16
Magic 2010: 8
Magic 2011: 2
This might be a good time to revisit a "Tales From the Pit" panel from Mark's column from Monday:
Well, guess what? Only one of these two cards is in Magic 2012. We're about to crown a winner in Core Set Survivor! So... to see the winner... of Core Set Survivor... please pause two minutes and watch some advertisements... and then come back here... congratulations... to the card... called Giant...
Congratulations to Giant Spider, the winner of Core Set Survivor! The only card, as of Magic 2012, to appear in every core set of Magic: The Gathering!