What's the number of cards per rarity in a regular set of MTG?
Modern Masters contains 229 cards (101 commons, 60 uncommons, 53 rares, and 15 mythic rares).

The Scars of Mirrodin set contains 249 cards (101 common, 60 uncommon, 53 rare, 15 mythic rare, and 20 basic land).

http://www.wizards.com/Magic/Magazine/Article.aspx?x=mtg/daily/feature/218