Obviously, it depends on the lengths of the songs you sing and how much chatting you do in between them. As with previous posters, my rule of thumb is 12 songs for a 45min set.

I usually have two additional songs ready in reserve should the gig go quicker than expected or I get asked to do an encore or couple more. I also earmark at least one song near the end of my set for possible deletion should I find myself running out of time. Nothing worse than keeping your big one for the finale only to have the emcee pull the plug on you after you finish the penultimate song on your set list.