if you plan on cloning in a large number, you might want to wait these seedlings out until they preflower on 18/6. This will give you the best option for the mother, pick the one that you think looks the best and keep her in veg. You'll have to get another space to keep her and the clones while your current grow finishes.

The amount of clones you can take depends on the size/health/strain of the mother. 20 or 30 from a decent size plant for sure. By that time, the prospective mother (that you're already growing) will be 2 or 3 times the size that it is right now.

I'm in the same kind of process that you are, trying to set myself up for a 60 plant SOG. I started 8 seeds, 4 turned out female and I cloned the best one as soon as it showed sex, about 6 weeks from germination. She turned out to be a great choice but I wish I would have just kept her as the original mother. There are plenty more options for you, this is just my suggestion. I hope you find one that works for you.

Good Luck...Good Grow