As much room as you can afford. If you are raising Coturnix, generally they need at least one square foot per bird. For the new world birds such as Bobwhites or Gambels and such, they need about 4 square feet per bird.
But, if you can give them twice or more of what is recommended for any of these species, the birds will be happier and healthier. Quail turn cannibalistic when crowded in too closely and the stress turns into fighting and injuries.
Make sure to keep the ratio of male to females proper, depending on what species you are raising.