This probably counts cells (i.e. a cell will be a service from an antenna facing in a specific direction broadcasting on a specific frequency) rather than towers. One tower can operate several cells--especially true for 3G and 4G where many carriers now do carrier aggregation across more than one frequency (and thus more than one cell). My local tower for one operator alone has 12x 3G cells (3 antennas facing N/SE/SW which each carries 2x 900 MHz carriers and 2x 2100 MHz carriers). For 4G the same 3 antennas carries 700 MHz, 1800 MHz, 2100 MHz and 2600 MHz services (for a total of 12 cells). So it adds up pretty quickly.