[Tom4784]

Trainers don't start using more than one until after Ceruleum City but I think the game up until after the second badge is like a tutorial. You can't fight Brock until you show the guy at the gym a water or grass type and you can't fight Misty unless you have a level 15 pokemon but no such restrictions are in place for Surge.

Trainers use more pokemon after you beat Misty and the levels jump up too.