You can seperate them whenever you want. I usually just keep them together in a large enclosure until the number of mantids is down to something I can manage. I have two groups of two different species that have been together since they hatched and they don't bother each other. Only a few were lost to cannibalism. Of course they are well fed. If you try to seperate a hundred nymphs into individual containers it's going to take you awhile to feed all those. Thats another reason I keep them together.
I separate mine at L1 into however many number groups as I want to end up mantids. So far the only losses I've had are due to mismolting... but Rick's right, it does take a lot of time. It takes me 1-2 hours to feed 50 mantids. It depends on how awake I am.
I always chuck them all into a large net, with loads of flies, just to reduce cannibalism by a LOT. I then wait until about L3, where cannibalism starts to kick in, and by this time, you are left with a handleable amount of nymphs, that are large enough to separate. Used that method for every single ooth I have incubated.