Team Total

You can have 2 Pokemon other than you and your partner on your team. First Chimecho Assembly has to open in the guild and I forgot when that happens. When you get to "Guild Sublevel 1", Chimecho will want to talk say that Chimecho Assembly has opened to you and your partner. There will be some dialogue and then Chimecho will ring the Friendship Bell and then POOF you can recruit new members just like that.

The max is four. Theres a maximum size as well. 6*'s is the max. Some Pokemon like Dialga and Lugia have 4* size so you can only have one of those 4*'s on your team and 2 1*'s, equaling 6*.

Oh, and the Pokemon you are and your partner are both half a *, so your team size is 1* at the beginning of the game. At one point, Chimecho, a fellow guild collegue, will open up Chimecho Assembly. Once you talk to her she'll ring the friendship bell and you'll be able to recruit team members.
One way to increase the chance of recruiting a member in a dungeon dates back to the personality test. There will be a question "Did you play Pokemon Mystery Dungeon: Explorers of Time?Darkness. If you answer yes, your chances of recruiting Pokemon are higher than if you say no.

P.S. I forgot when Chimecho Assembly opens and I will NOT, I repeat NOT start over for you so don't ask me to.

P.S.S. Sorry I forgot to put it in my previous reply.

Just a quick not-
Castform is a larger size than usual because he can change forms according to the weather.

4 altogether

4, but only if the total size stars are less than 8.

You can have 4 Pokemon on your team.