The existence of many moons above Carcosa is a common theme of the Carcosa Mythos. It is one of the features of Carcosa's celestial peculiarities, along with its twin suns and dark stars. However, it should be noted that where the city mentioned in the Play is not Carcosa, there may only be one moon.
There is an implication either that they hang incredibly low in the sky over Carcosa, or that its towers somehow soar beyond the atmosphere to vast, orbital heights, since it is said that the tower tops can appear to lie behind one or more of the moons.
In astronomy, planets with more than one natural satellite (moon) are very common, especially if the planet is further away from its star(s). While the Earth has one one satellite, the Moon, Mars has two, and the gas planets each have tens.