What do you mean by
You will not want to clip clones until the first true set of 5 prong leaves.
You mean I will not want to top them ? This brings me to another question:

I've been an outdoor grower for a couple years and I would always top my plants twice per season before they start flowering. When should I top my plants before putting them in a 12/12 light cycle ? Would toping them while in the 12/12 damage them or slow their growth ?

so if I'm getting what you are all saying, it does not matter how many plants I have under each lamps as long as I maximise the space, the yield should be similar ... (1 gramm per watts)

I use one 1000 watt hps. I use 1 gallon containers and use clones. I put 35-39 clones under light. I average just over 2 lbs of bud every 2.5 months
Do you think you would get the same yield if you had less plants of bigger size ?

How tall are you clones when they it 12/12 so that they all fit ?