A couple of these grew up in the tank from a spawn a year or so ago. So no, there is nothing special about 8. I started with 6, sold a trio off, and there are 3 originals left. I've since added a couple of pair from an earlier spawn too. I think the total is 9. Three or four in a 125G should be okay. Just make sure there is a fair amount of structure in there (driftwood, rocks, artificial plants) and you should be good to go.