If you add a Dword value named
it will change the upper limit in Mobile Hotspot, e.g. I changed it to decimal 10, and the Mobile Hotspot control panel showed "Devices connected: 0 of 10".
I have tested this with 10 connected clients concurrently.
I strongly suggest making the change with Mobile Hotspot turned off and rebooting for the changes to take effect.
The maximum value of this key appears to be decimal 128. Larger values prevent Mobile Hotspot from starting.