There are two limitations of PPTP in pfSense, due to limitations in the NAT capabilities of the underlying pf.
1. If the PPTP server is enabled, no clients behind the firewall can connect to a PPTP server on the Internet if they are NAT'ed to the same public IP as the PPTP server is using. If you have another public IP available, you can try to NAT the PPTP traffic destined for a specific remote server out that way. See this how-to.
2. Only one client can connect to a PPTP server on the Internet simultaneously. 10 clients can connect to 10 different servers, but only a single simultaneous connection can exist to a single server.