I suspect the usable number is much lower and would be just those satellites mostly opposite the Earth but with some signal reaching the moon? I recall the beam width of GPS antennas being like 30 deg (?), so almost all of the signal is directed at Earth.
They'll be using sidelobes. This is what's done at geo altitude which is also above the GPS orbits.
At geo the commercial satellite I worked with had position accuracy within about ten meters, and we always had access to 6-8 GPS satellites at a time. Obviously at the moon the signal is much fainter but my understanding is that it's essentially the same just harder to detect
