Apparently it's 155 engineers (if LI data is to be believed), and about an equal amount of Sales employees [0].
There's definitely value in the space, but I struggle to imagine how the ends (capturing that value) justifies the means (how they're pursuing that value).
If you know your way around CalDAV [1], you can build the most valuable 20% of their features in a weekend. A team of 3-4 could build the most valuable 80% in a month or two. In fact, some people have not only done that but kept going, and we have FOSS tools like https://cal.com as a result, that are arguably easier to use.
As a board member/founder/executive, one has to in the mirror each night, and feel confident saying "I'm running this company responsibly".
Especially if you're working in a purely digital space (which Calendly is), you have to bend really far to justify hiring that many people. IDK, maybe I'm a standard deviation or two away from norms on frugality, but this baffles me.
[0]: To contrast, WhatsApp sold in (!) 2014 (!) with 32 engineers (~50 total heacount), servicing 450mm active users. So, Calendly have 10x the headcount, on .2x the valuation.
[1]: https://en.wikipedia.org/wiki/CalDAV