Yep, you can fit as many games on as the card's capacity will allow*. When you turn on the DS the Simply boots to it's own menu and you select what you want to play from a list. Easy.

The new firmware even supports booting with cheats enabled, like Action Replay in a way.




*Remembering to leave 512k spare for each game, for the creation of the save file. This also makes backing up save files onto the PC drag n drop simplicity. ROMs of course vary in size, and can be trimmed to make them a bit smaller - I rarely bother with this, as it can (rarely) cause problems with stability or things like online or local multiplayer.