Routes 14 & 15 have a total of 16 trainers. They are spread out so if you are low on healthy Pokémon you can bypass them all if you go the right way. But you should always level up your Pokémon with Trainers because they'll give you more Experience than wild Pokémon.
The lookout station between Route 15 and Fuchsia City has another one of Professor Oak's Aides in it. He'll reward you with an Exp. All if you have over 50 Pokémon. Exp. All divides up Experience points to all your Pokémon after a battle even if they didn't do anything.
Next, on to Fuchsia City, where the Safari Zone and the Gym are situated.
