15-20 It is enough to do six groups a day in a group, and the interval between them should not exceed one minute. It's enough to exercise strength and endurance | Do 5-7 groups, each group does 2/3 of the limit, and rest between each group 1-2 minutes. You'd better do it sooner or later. How much you do depends on your ability. If you do this for a week, you will see that your biceps are getting bigger. If you persist in doing it for half a year, you will basically be better than the average person. But not too hard. If you push too hard, your muscles will ache the next day, and the pain will make you give up exercising. So you should do what you can and eat protein food every day, which is the source of increasing muscle.