Originally Posted by Fat zyrefredric
No, I'd say that for trainer battles, you may only use the exact number of Pokemon as the opponent. That means that if the opponent has three Pokemon, then you can only use three, resulting in a 3vs3 match, regardless whether you have six with you. If all three of your Pokemon faint before the opponent's team, you lose, even when you still have three other healthy Pokemon. This also means you can't have a trainer battle against somebody with five Pokemon when you only have four (non-fainted) with you.
This affects the trade-off you can make between how many Pokemon you have and how strong they are individually. Some people prefer just using their starter to kill everything, others keep a six-member team throughout, personally I've found a team of four works very well in-game. If you can't use more Pokemon than an NPC Trainer does then that swings things back in favour of small teams (with death fodder to make up the numbers so the Gym Leader will fight you), which I think Game Freak were trying to nudge players away from doing with the EXP changes in BW.
"If I may guess, Emboar might be an exception to the norm that the scalding liquid is expelled from the Pokémon's mouth" - Cobraroll, on Emboar learning Scald.
"Nothing like a cup of tea to start off the day" - Professor Oak, EP025.