For 3ds I had 3 slots for games and it was MORE than enough (Usually I had one of the fire emblem games, one of the MH games, some rhythm game (like Bit trip or Rhythm theif or final fantasy (the curtain call thing) and I had one slot for something that I currently fellt like playing (may it be Zelda or Mario Kart or even some Kingdom hearts)
My point is, if you have the right games, even two would be enough in my oppinion.