There's no standard assumption for the number of users, for that's dependent on the processing power of the router and many other factors. This is a complex subject which is why there are WiFi industry certifications for admins.
Given your use case of 50+ users in an office with at most one gypsum wall, I believe a more powerful router with more processing power than the Broadcom BCM47189 and BCM43217 processors in your existing ASUS router would be desirable. Also, a new router should conform to the new WiFi 6 802.11ax standard, and be capable of security with WPA2, WPA3, and 802.1X WPA Enterprise with a RADIUS Server. The latter may not be desirable now, but if you find later on you need that, you're prepared.
You can expand the network coverage with additional Wireless Access Points, or another router, later if you need to.