How many reps should you do per set? For many of my exercises, I tend to do 3-4 sets of as many reps as I can. For example, I might do 20 reps of a lat pulldown.
Obviously your total rep numbers will decrease after a few sets. Many beginners make the mistake of "getting their reps in" instead of going to near failure on their sets. For example, they might only do 12 reps when they could push themselves to 15.
This isn't to say that you can't count your reps. But it's better to make your reps count. Experiment with various rep ranges and see what works best for you!