How much bench presses should I do?

well i would say 20 pounds is ok for you to be starting off. since your looking to lose some chub i would be doing higher reps like 8-10 if you can it will help to burn more off than doing 3-5 reps lower reps are for building strength. it would help if you did some cardio or something else to get the heart rate up if you have a lot to lose cause its hard to just pick one spot and remove fat from it but doing high reps will tighten your chest up if that's what your looking for.