Current stats show 205,000k[1] towers - not base stations, I'd estimate closer to 410,000-840,000 mobile base stations. Figure most of those are not all four carriers on each site, some are just one carrier, some are all four, and if I were to pick a single number, 585,000. Previously, I'd suggest that you can really only compare the US with Canada, Australia in terms of population density and spread.

[1] http://www.statisticbrain.com/cell-phone-tower-statistics/