With the original A500 you can only have a 512KB trapdoor expansion to increase the ram. And depending on the version of the Agnus chip this trapdoor expansion would either add an extra 512kb slow ram, or would increase the chip ram to 1MB, which was required in later games.

With the later A500 Plus you could add a 1MB trap door expansion to increase the on board 1MB to 2MB chip ram.

For fast ram you need to either use a sidecar fast ram expansion, or other sidecar expansions that also added ram such as GVP harddrives. And some more recent fast ram expansions that piggyback directly over the agnus chip.