The Mystery Booster draws from a card pool of 1,815 unique cards from throughout Magic's history. In the Convention Edition, 121 of those are playtest cards which aren't legal for Constructed play.
Each Convention Edition booster contains 14 cards from the main Mystery Booster set, plus one extra-special “R&D” card.
Every slot in the pack has its own 121 card sheet that it pulls from. So, every pack will break down like this:
2 White commons or uncommons 2 Blue commons or uncommons 2 Black commons or uncommons 2 Red commons or uncommons 2 Green commons or uncommons 1 Multicolored common or uncommon 1 Artifact/land common or uncommon 1 Pre-M15 card 1 Post-M15 rare or mythic 1 Playtest card (Convention Edition)