How do you determine your main if you master multiple characters?

The one you enjoy playing most from these you master ^^

just play whatever champion is approriate for yours/the enemies' team comp. if it's normals, play whatever you feel like.

It is possible to have multiple mains, it's just less common. A main is simply a champion who takes up most of your time when it comes to practice, so if you have multiple you are attempting to master you can consider them all mains.