Quail fly really well. So they are raised in cages, or pens with roofs.
The number of quail will be limited by the amount of pens you want to make and take care of.
Unless you intend to sell quail, you should have no trouble raising as many as you want on an acre.
Several people have already answered about how many can have enough room to move around.
But chicken manure is another consideration.
https://en.wikipedia.org/wiki/Yarding
Wikipedia quotes the book "Poultry Breeding and Management" by James Dryden (published 1916), which says 100 hens per acre...