One strain at a time in a tent. You have to think about what will work for your grow style. Most strains have some info available somewhere. From seed, I'd grow out four at a time.
Almost every strain has phenotype variations, so I take cuttings of each plant before flipping to flower. Usually, 4-5 cuttings per plant will produce a couple good clones of each. So as the cuttings root and I get them into pots vegging, the original four plants are flowering. Throughout that process I watch each one and pay attention to their growth characteristics for things like node spacing, growth rate, and flower time. When they finish flowering, I sample each one for effects and make a decision based on everything each plant has to offer.
So now I have an exact clone of the plant I want to be a mother, mature enough to take cuttings from. So, you root some cuttings and you're off to the races. The biggest advantages for me are having even growth rates, canopy height, and nutrient requirements.
A separate grow space is required to do this, I do mine in a divided tent. The tent is 4'x3', with a reflective barrier to divide it. It gives me a 3'x3' space for moms, and a 1'x3' space with a shelf for cloning. It could probably be done in a smaller space; you would just have to train and maintain your moms for it. Repotting and managing the height makes it work.
Hope this helps.