How many reps?

The number of reps you do in a set depends upon your training goal. If you're looking to build endurance, then do 12-20 reps with a weight approximately 60-70% of your max. If you are training for hypertrophy (adding muscle mass), then do 8-10 reps with a weight approximately 70-85% of your max. If your goal is to build maximum strength, then do 4-6 reps with a with approximately 85-100% of your max.