You need powers of 2 for it to work
It needs to constantly break down to another power of 2, or else someone's left without someone to face and cannot proceed.
Only possible bracket numbers are 2, 4, 8, 16, 32, 64, 128, 256, so on so forth. Otherwise, there's a kink in the matches.
I was thinking about doing qualifying rounds, but the biggest issue with that is time
Anyway; if I'm heading up the next tourney, I may just do a ladder (as mentioned before). It's more time consuming to make, but it allows any number of contestants.