As said before, for grass, Pokemon 1&2=20%, 3-6=10%, 7&8=5%, 9&10=4%, 11&12=1%.
You can have the same pokemon in multiple slots, so for example if you have the same pokemon in slots 1, 3, 8 and 11 then that pokemon will have a 20% + 10% + 5% + 1% = 36% chance of appearing.
However, I figured...