If the carrier were the size of the moon, there'd only ever need to be ONE. The only ship the size of a celestial body is the alien mothership (which is much bigger than the moon).
In an earlier conversation with Winter, we agreed that a carrier should carry about 8 smaller UFOs. You don't have to stick to that figure, I could see it carry any number up to 20 or so. But it even then it would be MUCH smaller than the moon.