I am not sure about taking screenshot and checking pixels in resulting texture (most likely will need C++).
However if that can be doable you can create postprocess material that shows only pokemons visible part (as white with black background). Very similar to any outline materials (and you can find tutorials for that). Then make snapshot/picture with that material applied (you can hide it in fake flash effect etc.)
So when you have that scene capture material, you can calculate white pixels or simply calculate average brightness of whole frame.