How much storage do I need?

It depends on the library, not a rule of thumb. Retro libraries are compact — even a large collection fits comfortably on a mid-size card. PC-handheld native games are the opposite: modern AAA titles routinely run 50–100GB-plus each, so a 512GB or 1TB drive fills fast. The practical answer is fast onboard storage for native games plus a genuine, appropriately sized microSD for the rest. Buy real capacity from a trusted seller and verify it, because counterfeit high-capacity cards that fail on fill are the classic way to lose a library.