+1 to @MerakiDave. Even if the GUI allowed you to create 10,000 users, the GUI would be horrible at managing them. It was simply not designed to run at that kind of scale.
You absolutely want to go to a RADIUS based solution. If you wanted a Cisco solution then you would use Cisco ISE.
https://www.cisco.com/c/en_au/products/security/identity-services-engine/index.html
If you want something free you could consider FreeRADIUS:
https://freeradius.org/
Potentially there might be some third party cloud providers doing hosted RADIUS as well.
You could always put a RADIUS server into something like Amazon AWS if you don't want to deploy any infrastructure.