I was curious to see how much I could detect with an off the shelf setup (standard PCIe wifi card with an external 10cm antenna) running in my living room for a week. Here are the numbers so far:

Raw size: 2,077,031,816 bytes (2.0Gb) (14,991,993 lines of tcpdump data)
2019-09-11 22:21 to 2019-09-19 06:13 (approx 7 days, 7 hours and 52 minutes) 10,554 minutes
Unique MAC addresses seen: 68,298

Unfiltered raw log data. Notice the peaks at 8am and 5pm on weekdays.

• There are about 146 devices permanently 'on' near to the antenna (I saw their signal consistently 10% of the scanning time which corresponds to the scan frequency at the time - I was changing channel up to 10 times a second).
• 30 of these 146 devices I saw 90% of the time despite channel hopping, so they were either putting out enough packets to get on the list or they are routers on multiple frequencies or just near enough to the antenna they are always heard first.

Wifi devices per hour

Wifi devices per 30 minutes

Wifi devices per hour (background devices removed)

Wifi devices per 30 minutes (background devices removed)