MAC address tracking is a good simple way to get an approximate number of people: it's very easy to install, it requires only a WiFi antenna, and the data is easy to translate into a count. However, there are privacy and legal concerns which have prompted phone vendors to obfuscate this data. And there can easily be zero (or more than one) broadcasted MAC address per human, even when filtering by OUI.
Retailers have been using specialized "people count" technology like infrared break-beams, thermal cameras, and CCTV+CV systems for a long time. CCTV is the most accurate by far, but it's also commonly considered to be an invasive level of surveillance (and rightly so). It's also not particularly accurate in adverse situations. At Density, we looked at (and tried) many of these technologies - but ultimately found them lacking because they were either too invasive or too inaccurate. The device we've built uses a lower-resolution depth-only sensor because it can return extremely accurate results, without having the capability for facial recognition or other analysis techniques that are harmful to privacy. So far the technology is working very well - with an algorithm based on a deep-learning "human classifier" we're seeing accuracy above 99% in many of our deployments.
Here's a cool (and in-progress, excuse our dust) summary of some issues that the technology has to navigate: https://faq.density.io/algo/