I want to create a Private Tournament, but I don't know how many players will join. If I set it up for 20 players and groups of 2 - can I start the tournament if only 12 sign up? Or do I need to know an exact number of participants before I create the tournament?

You will need a full tournament to start a private tournament, so pick a reasonable number of entrants initially.

You can easily remove an unstarted tournament though, and recreate it if you need to. (Everyone will then need to re-enter though)

Excellent - thank you