multiple nitrogen trees growing new babies after tree being cut down

New to permaculture and have lots of baby trees coming up and know they are nitrogen legume trees.  How long do I leave them growing before cutting down for the mulch and do I thin them out before they grow up.

Everything you just said depends on the tree.

I would say cut them down at the same point you cut down the parent tree. As for thinning, you'll probably want to thin them out when you can tell which ones are going to do the best, but I really can't tell you much without knowing, at the very least, what species of tree it is.