Raise maximum connection limit for Internet Sharing?
Unfortunately there's no setting to configure the maximum number of stations that can connect to the access point at a time.
The limit seems to be hard-coded into macOS and cannot be modified by setting properties or similar.
However, if it is an option to use additional hardware, you may put a router in between. That way, only the router would connect via Internet sharing, while the other devices would connect to the router.