It really depends on the exercise for me.
At one end, I'll do pyramids for the big lifts. So for example, when deadlifting I'll do 10, 5, 3, 1, 3, 5, 8. So 7 sets.
At the far other end, if I'm doing something like tricep pushdowns, I'll probably just do 2 sets of 15.
For dumbbell shoulder press, I'll do 12, 12, 8.
For reverse military press, I'll do 4 sets of 10.
And even then, some workouts I might do 15, 10, 6 for dumbbell shoulder press.
So for each exercise, I have pretty much a unique set/rep design, and on top of that I periodically change them up individually.
It sounds crazy and much, much more complicated than just doing 3x10 for everything (for example). But really, it's all what works best for you. I take a journal to the gym, so I remember it all. But ya, again mine's pretty all over the place.