How many transmission stations for a single cell phone company in USA?

Answer 1

Every cell phone company in the US has a different number of transmission towers. There is no number that answers this question accurately.

Answer 2

Hundreds and thousands. You haven't specified enough to give a better answer. In an urban area dozens are required to service many users. In a rural area much higher towers are used to cover a bigger area with fewer users, maybe 1 per mile or 2 of Interstate. And if you look at a coverage map, there are places where only along the road is covered.
http://www.rentcell.com/coverage-map-att.htm