Mostly two tunes.
With a bit of luck, a set dance will go round exactly twice and we change tunes halfway i.e. when the first couple start again. Most of our favourite callers like to go exactly twice though - once isn't enough, especially if the dancers are young and fit, and once and a half isn't fair on the couples who don't get a 2nd go at being top, though we've worked with callers who do both of those things routinely.

For 64 bar dances, we like to use a 64 bar tune (they do exist) and stick with the one tune. In the case of Cumberland Square Eight, that's flying in the face of convention, but you really don't have to play My Love She's But a Lassie Yet followed by Atholl Highlanders. We use either The Silverton Polka or The Weymouth Quickstep all the way through.

We also have lots of 48 bar jigs now. They are all cracking tunes and I can hardly believe how much I used (decades ago) to play 32 bar tunes AABBAB when 48 bars were required.

(that's enough trade secrets)