There are 16 gym leaders for you to face. 8 in the johto region, and 8 in kanto.
Johto
____
Falkner-Flying
Bugzy-Bug
whitney-Normal
Morty-Ghost
Chuck-Fighting
Jasmine-Steel
Pryce-Ice
Clair-Dragon
Kanto
__
Brock-rock
Misty-Water
Lt. Surge- electric
Erika-Grass
Janine-poison
Sabrina-Psychic
Blaine-Fire
Blue-Mixed
hope this helps.