Don't let people bailing on your trip dissuade you from going. You should go for it. As for minimums, its subjective. My smallest multiday was a 5-day Salt trip with a crew of 3 (me rowing + 2 kayakers) and we had a great time. The minimum depends on what your confidence is with running big volume water like the grand, and what kind of trip you're looking for. I certainly like to boat with a big crew and go fat if I can, but some people even like to solo the canyon, and it happens more than you would think (At Lava last trip, I met a kayaker on an 8 day solo self-support trip). There's a light, but steady stream of river trafic down there even in winter, so if you were nervous about the biggest rapids you could possibly link up with other groups and boat in numbers for safety if you had to.
Good luck, and have a great trip!