OK here's a tally of the types:
Normal = 3
Fire = 2
Water = 3
Electric = 4
Grass = 2
Ice = 3
Fighting = 3
Poison = 1
Ground = 2
Flying = 3
Psychic = 2
Bug = 3
Rock = 3
Ghost = 2
Dragon = 2
Dark = 0
Steel = 2
So Electric is the highest. However, there is also the Fighting Dojo in Kanto which would make 4 for Fighting. Also in Black/White the first gym can be Fire, Water or Grass, making 4 for Water too.
EDIT: just testing something :)