Kind of a broad question... no emulator requires a set number of buttons. It's the individual games that make these demands.
If you're limited to seven buttons (does that include any start buttons? coin buttons? is there a keyboard handy?) you've good up to about SNES (if you don't need "select.") You're limited with Colecovision, Intellivision and Jaguar games that need a keypad to play (I know that's all of 'em for Coleco, the keypad starts the game.) PSX and newer consoles: you'll have a bit of limitation.
What exactly are you looking to play? Start with that.