Clockwise supports the ability to automatically scrape participant count from a specified zoom meeting by passing the zoom url through the --url / -u flag.
Alternatively, you can leverage manual input mode, which collects user input via the TUI to manage participant count. Just exclude the --url flag.
This can be approximate, but you can use resources like indeed.com, glassdoor.com, and levels.fyi to be methodical in your approximation.
In conjunction with OBS, Clockwise can print your total meeting cost as it increases in real time to a virtual camera. The virtual camera can then be consumed by Zoom to passive aggressively remind everyone precisely how much a given meeting costs. Anecdotally, this raised awareness and led to more focused and less frequent meetings for my team. To change something, track it.
OBS configuration is a bit involved, but it's something that you only need to do once. Long term, it would be nice to replace OBS with something like FFMPEG that doesn't depend on an external GUI.