xmas is soon: Adding 12 as an option sounds kind of bogus to me. I think there are two reasonable options:

1) Keep the current system. In this case, if you know you won't get 16 players, you'll have to set up a tournament for 8 players. But everybody has the same amount of matches to play.

2) Change it to what kleineme suggested: Every tournament can be started, and if the number of players is not a power of 2, pick those who advance to the second round randomly, let the rest play it out. Or you could even pick them by BKR (if the tournament isn't random, hm )