I go by the rule of thumb: 20 gallons for the first gecko, 10 gallons for each gecko there after.
So in my 55 gallon, I will be housing four girls in the end.
If I had a 75 gallon, I'd house up to six females or 5 females and a breeder male for a term.
If you are going to house one male with multiple females- I'd give at least two layboxes so multiple females can lay around the same time and it may help you find out which gecko mothered the eggs. If you care about that.