It seems complex at first, but really isn't.
You get one default DXi/VSTi synth. This is for basic use and is assigned to the last port (32 or DXi port 16) when you select the "Reroute all MIDI to default DXi port. If you don't do this I *think* the program then considers whatever DXi synth is being used is the default, which would be the one on port 17 (or the first DXi port)

The reason for ports is so you can route a MIDI track to a specific synth. You can choose up to 16 different synths by using 17-32 port numbers. You can even open a second instance of the same synth on a different port, and the changes made to it will not affect the first instance. Very handy. Most systems would not run more than 16 DXi/VSi synths at once so 16 ports is plenty.

As mentioned, the first 16 ports are for selecting different MIDI outputs on your computer to external devices, either synths or effects usualy (or even light shows). Most people only have one or two external ports so again the 16 ports (1-16) are plenty.

I do not work here, but the benefits are still awesome
Make your sound your own!