You can play two tunes for each dance, but I recommend three-tune sets because the tunes don't go on for too long, and because you'll occasionally intend to play three but only get do two.
Having a three-tune set collapse into a two tune set isn't bad. Having a two-tune set collapse into a one-tune set is a pain.
There are several reasons it happens:
Sometimes the fiddler can't remember how to start a tune. This even happens with tunes you know well. At one dance, the third tune in the set was a tune I'd written, and I couldn't remember how to start it. If you have another tune you can switch to, fine. If you have two other, good tunes, you can just stick with them.
Sometimes, tunes don't work. A caller will turn around and say, "Switch tunes," because the dancers can't hear the phrasing, or because the tempo's wrong, or because the tune just doesn't fit the dance for some other reason.
Sometimes, the caller won't notice, but you will. You'll discover you've picked a crooked tune or a tune that's half-length, or you've tried to shorten a three-part tune but keep starting the third part out of habit. Say, "Switch."
Sometimes, you'll switch tunes too late, or the caller will decide to make it a short set. You'll be ready to switch tunes the next time through when the caller turns around and holds up two fingers to tell you, "Two more times through." Rescind your decision and don't switch.
Sometimes, you'll choose tunes that sound too much alike, switch into the second of the two, and find yourself falling back into the first. I played one dance with Elizabeth Stevens at which she fell back into the same tune three sets in a row. The rest of us were laughing so hard it was a challenge to keep playing.
Four tune sets don't give you enough time for each tune. Three tunes supply the happy, Goldilocks mean: two's too cold, four's too hot, three's just right.
