Depends on the games you have digitally, or want to get. If you take the biggest games (20GB) those will fit ~25 times on a 512GB disk (actual available disk space is a bit less).I plan on getting one for my Switch soon. Not sure what capacity
I looked at size and speed, those Samsung Evo cards are relatively fast and relatively cheap. You can get them cheaper but they'll be less quick due to it, 60MB/s (UHS-I) is minimum speed for the switch so that should still work fine.