Common
~30 cards (approx.)
The most plentiful rarity and the backbone of most collections. Commons drop the most frequently, so they are the easiest cards to level all the way up.
Upgrade path
Starts at Level 1 and maxes at Level 16 (16 levels — the longest track). Each level-up needs the most duplicate cards of any rarity, but those duplicates are also the easiest to accumulate; gold cost climbs steeply toward the top levels.