How many legally backed up roms will fit and play on an sd card with an m3 adapter. I want to leave enough space to save games also.
Depends completely on the games since they range anywhere from 7 megabytes (Brain Age) to 115 megabytes (Bleach DS).
Back-ups or saves? Cause save games only take up 256KB each, and as long as you use the game manager you shouldn't have to worry about space. It'll let you know if it's full or not.