You'll never find a clear and definitive answer on this. Some people swear by 10-12, others 4-6 and some even higher than that. I'm using 4-6 right now for some great gains in size on a bulk, but I also used 12-15 for great results on a cut/maintenance.
Also depends on body parts too; some muscle groups need a little more attention. I like a "break" from things for a week every 12 (but that's just me). As far as switching it up; base it on your results visually and progress in terms of reps/weight.