Hi,
Coturnix will require 1 square foot per bird, so a cage 2ft X 2ft will hold 4 birds correctly. A cage 2ft X 2½ft is 5 square feet so five birds.
I don't always follow this rule though. In the hight of the breeding season I might add one or two more to a 4 or 5 square foot cage to make decent breeding groups up. As long as they are not over crowded I find this works.
Solid floor cages can be stacked to whatever hight you require and are comfortable working with safely. Cages only need to be about 12-14inches high so you could safely have 4/5 cages high. Make sure the top cage is at eye level so egg collection is made easy for you to see in all the corners.
If possable, have food and water on the outside of the cage, this will allow more space inside the cage and can be easier to supply food and water.
Ironsun.