All of them. Everyone starts with C, but you really need a set if you want to be versitile. My very limited knowledge of harp is as such, straight forward rock,folk and such, generally, say the song is in A, you play an A harp and start on an exhale. With blues, say it's the key of E, you have a harp that's the 4 chord, in this case an A, and start on a draw. Of course, this is grossly oversimplified, but hopefully helpful.Where... Does one get a setup on a harmonica? I'd like to play more... The current one is in C. Is there a better key to get?
I really suck at harp though. So there's that.