I would think 75-78 would be fine. Feed them heavy and pull your mops everyday. They only lay a few eggs a day. You should have plenty of production pretty easily. Another way is to do it in a tank full of java moss. Let it grow nearly to the top. Then you can move the breeders after a week or two, and you will see fry darting around the moss eventually. IME rainbows are slow to hatch. Make sure your water is hard. How many breeders are you running?
