A full team is usually considered to be about six Pokemon--that's the limit people are allowed to battle with at a time. But there are plenty of trainers that catch more than six, and will occasionally rotate them out with their current party to bring with them whichever Pokemon they need at the time, while the rest of their Pokemon are looked after by the professor of their region or at their home while they're gone. My friend Ash, for example, had at least nine or more Pokemon that he's acquired during his travels in Unova. [And dozens more from other regions, for that matter.]

As for myself, Pansage, Crustle, and Stunfisk are the only Pokemon I have. [So...unless Cilan catches a fourth Pokemon or what not at some point in the future, he doesn't have to worry about a team member being left behind at the moment.]