At this point I can do 8
instruments via the Unitor but I'd like to at least double that
capablilty in the very near future.
Sweetwater just an email about price drops on all the Mackie mixers and it easy enough to pick one up used. The Unitor/AMT8 manual says you can link a lot of them together - i think 8 or 16 units. There is a diagram in the manual of this. I'm not sure about the driver/setup situation for windows though or if that older version of Logic handles multiple midi interfaces.
One thing you can do without getting another Unitor/AMT is simply use the MIDI throughs on your exisiting instruments. That can double your set up without buying any new equipment.