An estimate of how many satellites are needed to serve 500,000 users in the continental United States would be 500000 / (8x296) = 211 satellites when using UT-1.

With a uniform distribution of satellites over the Earth’s surface, and taking into account only the U.S. territory (2.55 percent of the Earth’s surface),the constellation should include 315 / 2.55% = 8,274 satellites.

in 2021, there were 1200 of them

Starlink is a fantastic service that I have been using for years with high speed and high reliability.