The obvious question here is how long are your levels? Also how challenging is your game and do you have enough new ideas/ themes/ mechanics to keep the player's interest if you do add more levels?
Viewing post in Levels?
Ok, I'd say if you're going for short and simple then 12's probably enough, especially as it's nearly Xmas.
Suppose it also depends on whether you think the game will greatly benefit from additional levels, or whether they'd just be to pad things out a bit. If the second one, definitely release it now.