There isn't one answer since there are so many factors. It all depends on bandwidth used, speed/bandwidth of clients, and environment/interference. I usually suggest around 20 clients before getting another access point added (and to use Ethernet whenever possible). Cameras and video streaming use the most bandwidth.
The 842 is 300Mbps (2.4GHz) and 866Mbps (5GHz). Remember WiFI is shared bandwidth so it does not act like an Ethernet switch where each port has dedicated bandwidth. So if you have 20 clients using the 2.4GHz band, they all can use a piece of the 300. Each client does not get 300.