That's a recommendation based on hardware capabilities of the router to support client traffic, however they're attached.
I have an MX64W that sometimes has up to 80 devices connected (wired + wireless) without any issues but that's only when there's events or meetings happening at that location. If you require more capacity you'll probably need to step up to an MX84.
If you're wondering about WiFi client capacity, that's based on a different set of factors. As a general rule I try not to have more than 20 clients on a single access point (and no more than 10m away), but you can get away with more clients depending on the wireless technology, the frequency, and the number of radios.