Hello Steve,
I'm not aware of any theoretical maximum on SSLVPN connections. I would assume that if 20 is the amount listed, that there is probably a hardware/software/usuability issue that caps the SSLVPN connections at 20. I would not advise exceeding that maximum as it would be an unsupported configuration.
The PC switchport on the phone is usable when using the SSLVPN connection on the phone. However, the PC connected to that switchport is not connected to the VPN. So the PC would also need to VPN into the HQ network.
Let me know if you have additional questions regarding this.
Thanks,
-john