I would think one roo would be happy enough, but you might not have a real high fertility rate. How old is the roo?
Check your eggs and see if they are getting fertilized. the recommended is 10 hens to a roo, so you really need 4 more hens, to get one more roo. (Is that chicken math?) But you might also get a wreck going with roo's fighting if you try and add another one.