For the purpose of authentication protocols, IP address assignment, and so forth, the VPN ports use the exact same server properties as those used by dial-in clients, so I will not repeat them here. Because of this, I will only cover settings relating specifically to the configuration of VPN ports in this section.
You probably noticed that by default there were 10 VPN ports automatically configured when RRAS was started, providing 5 PPTP and 5 L2TP ports by default. Since a VPN connection will be coming in over a network card, in theory the number of possible incoming connections is only limited by the processing capabilities of the system, and not by physical ports. However, the maximum number of ports supported is 30,000 for a given type (such a PPTP WAN Miniports for example).
Note that you cannot change the number of ports to 0, even though the system suggests this is possible. At a minimum, you must have one of each port type available. The reason I mention this is because you will need to configure how many of each type of port you wish to have available for connections, as well as which protocol they will use. If you had chosen to use PPTP for VPN connections for example, it would make sense not to allow incoming connections via L2TP. This would be controlled not by setting the number of L2TP ports to 0, but instead by configuring the L2TP WAN Miniport properties to not allow incoming connection.
So why would you choose PPTP over L2TP or vice versa? The answer depends on the level of security you require, as well as the security mechanisms that your network supports. For example, PPTP supports only user-level authentication, meaning that any connection using PPTP that has the correct username/password combination will be allowed. In contrast, L2TP requires 2 levels of authentication – first the machine is authenticated (via a machine certificate that would need to be pre-installed either via group policy or using certificate services) and then the user is authenticated using PPP. This allows a higher degree of security, since both the user and machine must be validated. The downside is the extra effort involved with using L2TP, as well as the fact that only Windows 2000 has built-in L2TP and IPSec capabilities among Microsoft operating systems.