Digital Performer allows for multiple midi channels per track. This was designed during the days of hardware synths and samplers.
Since the arrival of virtual instruments, there have been many changes in technological concepts-- such as VSL and its approach of allowing multiple articulations in a SINGLE matrix on a SINGLE MIDI channel. This great simplified the need, for example... in the First Edition when used with Logic, to have each articulation for a single instrument loaded into lots of *separate* instances of EXS24. Now, one single instance of VSL can host over 100 articulations (144, I believe, with some consideration to one's RAM and hardware limitations).
As far as Digital Performer goes -- in the context of MIDI-- there's no need to be concerned about there bing 16 MIDI channels *assignable*. The important thing is that each instance of VSL can only receive on one MIDI channel, no matter what features any particular DAW may offer.
You may assign any MIDI channel in DP to control VSL, but generally, each instance will work quite well with each assigned to MIDI channel 1.
Because of the way VSL works, I guess it's safe to say in this case "it doesn't make any difference which MIDI channel is used". If you have four instances and they are all assigned to MIDI Channel 1, keep in mind that these are all different MIDI Channels 1 because each instance of VSL is different.
You will see:
Vienna Instruments 1-1, 1-2, 1-3.... up to 1-16
Vienna Instruments 2-1, 2-2, 2-3.... up to 2-16
Vienna Instruments 3-1, 3-2, 3-3... up to 3-16
.... and so on. The point is that MIDI channel 1 on the first VSL instance is "not the same" as MIDI channel 1 on subsequent instances.
One reason why it's a good idea to see at least 16 MIDI channels in DP is that other VIs make very good use of those 16 MIDI channels, like Kontakt. However, DP will still "publish" (or "show") all available MIDI channels in its system whether or not a virtual instrument can make use of all of them--- because not all virtual instruments are set up to work with multiple MIDI channels in a single instance.
Hope that helps.