General help on topping plants.

Howdy, I learned that topping plants is a great way to have more marijuana buds and for them to stay shorter, and more bushy. I have around 20 plants that have made their way from clones to my infant room. I have a 100 watt fluorescent multiple light holder, about 2 feet from the tops of the plants. My plants now are about a foot tall each. I have recently topped them for the second time. My question is how many times/how often can you top them, and does topping effect yield.
 
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