Hello,
Thank you for reporting this.
It seems to be a WebUi validation issue. You can try adding additional clients by editing pptpd file in /etc/config/ directory within the router via some text editor in CLI/SSH, or, I found it slightly faster, by logging to the device with WinSCP and modifying the file mentioned above.
As for the user count, RUT950 should support up to 100 users.
Best regards,