Abstract: Methods, apparatus, systems and articles of manufacture are disclosed to determine the number of people in an area. An example method includes collecting media identification information corresponding to media presented at a location, responding to a received request to register a mobile device with a base station by storing device identification information for the mobile device at the base station, timestamping the device identification information, determining a number of mobile devices located in an area based on the stored device identification information, and identifying a number of persons exposed to the media based on the media identification information and the device identification information.