How do you know how many reps to do?

This largely depends on your goals. If you are seeking to build muscle then the science shows that hypertrophy occurs at the 10-12 rep range and this is typically what I have clients do for the first 60 days. After that to make the newly grown muscle fibers as strong as possible I drop the rep range to 8-6-4. This allows us to cycle back up to the 10-12 rep range to again continue building muscle at an alarming speed! 🙂