There are definitely only 10 teams, atleast for the first season.
As for "popularity in battling", I guess you can say that. I would say its mostly how good you are, people know all the good battlers and such and your ladder rating can help.
For the world cup, teams held tryouts where people kind of "proved" their worth, so to speak, so managers might do the same. We'll see what the managers do once they've been decided.