Note that over time, as your muscles strengthen and get used to the strain you put on them during exercise, they will also not grow any more. For this reason it's recommended to increase the weight over time, as you get stronger and your muscles grow bigger. For example, if you can do 20 push-ups now, that's all good, but don't expect to get big arms by working your strength up to do 100 push-ups. Most people would recommend a series of 4-12 reps in 3-5 sets for each exercise, and gradually increase the load as your body grows stronger. Popular combinations are 4x6, 4x8, 5x5 and 3x10. So instead of increasing the number of push-ups you do, increase the load instead (i.e. by doing push-ups with a downwards incline or with a weight on your back). That being, if muscle growth (size) is your goal.