If you don't want to cycle through alot of roms, get a 16GB or 32GB microSD, the 32GB one will still require alot of cycling. Anything larger will require a few minutes of cycling if you have alot of roms you want to carry with you.
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You will want to have about 512KB set aside per rom, this is for the save file. Can reach upto 4MB on newer roms. Pokemon roms save the save data on the writeable part of the ROM, you will want to make sure you have enough space like a normal save file.