After writing all of that information up, it is still missing what the actual bug or feature request is. I assume that you want MIDI import to support an arbitrary number of MIDI ports rather than the current maximum of two, and you already give a good answer yourself why that feature won't be needed:

Of course, I don't think anyone ever did THAT

The problem with allowing an arbitrary number of MIDI ports is that OpenMPT will run out of pattern channels to assign to these different ports very quickly, because each pattern channel belongs to a specific MIDI channel. It's not uncommon that a regular MIDI import with a single MIDI port already takes up 50+ channels, and with 2 MIDI ports it is already very likely to scratch the limits of the maximum number of pattern channels. Going even higher is just going to make that even more likely, and instead of importing more MIDI channels, you will actually end up importing fewer channels.

So: Unless there is a very, very good reason to support those extra MIDI ports, I don't think this going to happen.

Ultimately, the number I was asking for was 4, to match the MU2000EX and SC-8850. I'm sorry if my wording was a bit off. My apologies.

To put it simply, my request is that the max be 4, since that's as far as hardware synths went.

Yeah but why, what are you trying to achieve with this? If your only argument is "some hardware could do this", there is about a hundred other features not supported by MIDI import that "some hardware" at some point supported.