If you can complete more then 20 repetitions three sets with your current weight you should

Something that you should consider doing in this situation is adding more weight to your repetitions in order to make it more challenging, so that you build more muscle mass by pushing your body a bit further with an increasingly difficult exercise routine.

If it's a smaller child or infant, place one on the middle of the chest and on the child's back. (if it is a bigger child with enough room for two, place one on the upper right/your left of the chest, and one on the lower left/your right) the pads should never touch each other. 

The answer is either 3 or 4