One good guideline here for an everyday encounter is to multiply the average Pokémon Level of your PCs by 2 and use that as a projected baseline Experience drop per player for the encounter. This results in about enough EXP for each player to get a Pokémon half a Level from the encounter. This assumes an encounter with a Significance Modifier of 1x, or insignificant. A Significance Modifier of 2x, or the lower end for an everyday encounter, will double your baseline and nets you about enough Experience for a Pokémon to Level Up once. As you create a more important encounter, not only should you increase the Significance Multiplier, but you should also increase the baseline Experience you're working with, as that will determine the Levels of the enemies you create.

From there, simply mutiply the Experience drop by your number of Trainers. This is the number of Levels you have to work with to build your encounter.

Example: A GM is designing an encounter for three Level 10 Trainers, each with Pokémon around Level 20. Multiply the average Pokémon Level by 2 for a baseline Experience drop of 40. Multiply this by 3 for the number of Trainers for a total of 120 levels to build an encounter. The GM splits this six ways and stats up an encounter with six Level 20 Pokémon. If the battle has an everyday Significance modifier of 2x, then each player gets 80 Experience and can approximately Level Up their Pokémon once from the battle.

For a bigger, more important, fight, the GM decides on a baseline Experience drop per player of 60 and a Significance Multiplier of 5x for a total of 300 Experience, enough to give several Pokémon a Level or give one or two Pokémon a couple of Levels. Multiplying 60 by 3 for the number of Trainers, the GM has 180 Levels to build an encounter. He splits it into two Level 40 Pokémon and four Level 25 Pokémon to support them. He could also give up two weaker Pokémon to make a Level 25 Trainer.