I have a show that's about to go on tour.
You probably already know this, but I'll state this for the benefit of anyone else who may read this reply in future: My #1 advice is to bench test all of your hardware for your setup as soon as possible; do not wait until you are at your first tour venue to check if your setup works the way you think it will. If you can, rent or borrow the hardware you're thinking of buying for a couple of days, put everything together, and ensure it does what you need it to do before you go on tour. If you end up buying the hardware without being able to try it first, bench test your hardware and setup by putting it all together and making sure that it does what you need it to do before you go on tour. Tell whoever's in charge of the finances for the show that this is essential in order to make sure that load-in and tech goes smoothly in the new format. They need to make room in the budget and time in the schedule for you to sit down with all of this hardware and set it up in an environment where you can test it before you arrive at the first tour stop and, ideally, as soon as possible, so that if you discover a problem, you have as much time as possible to order new hardware and do more testing. If this means renting you six projectors, buying you the hardware you think you need, and renting a rehearsal space so that you can set everything up, then that is what needs to happen. Especially due to the chip shortage caused by the pandemic, you do not want to find yourself in the position of loading in three days before the performance, only to find out that you need hardware that can't arrive faster than two weeks because of stock shortage/shipping/etc.
I'm trying to run this ALL of my MBP (Intel)...so I'm going from 3 projectors to 6 projectors.
While it does depend on the wrapper, codec, and resolution of the video files, the resolution of the displays you're using, and the number of video files that you'll be playing simultaneously, my first reaction is that this is a LOT to ask of an intel MBP.
Do I need to buy a Matrox quad2go ($$$!) or is it possible to send the quad signal out of two HDMI splitters
If you need individual control of the content, blending, and mapping for each of the six projectors, you will need something like a Matrox Quad so that your computer and Isadora will register each projector as its own display.
If you are putting the same content on some of them and don't need to adjust the blend and mapping for each display, you can get away with using splitters (e.g. if projectors 1&2 don't use a splitter so they can be addressed individually [and can therefore be blended together & have separate mappings], projectors 3&4 come out of splitter 1 and don't need blending nor mapping and only need to show the same content as each other the whole time, and projectors 5&6 come out of splitter 2 and don't need blending nor mapping and only need to show the same content as each other the whole time. A splitter cannot replace something like a Matrox because a splitter is seen by the computer as a single display; a splitter doesn't allow the computer to see and individually address the displays that are "downstream" from the splitter as separate displays, so two projectors coming out of one splitter will always be treated as the same display, (i.e. show the same content and have the same blending/mapping).