I was in a 4 plane partnership on a C 180 on floats and I am now in a 2 person partnership on a Nanchang CJ6A.
I left the first partnership after 3 years mostly due to all the whining over the cost of maintenance. The other three partners were lower time PPL's and I carefully explained the realities of airplane ownership before we bought the plane but they still moaned and dripped about everything that needed to be fixed.
My current partner and I think alike about maintenance and we have a highly successful partnership. I think a 2 person partnership is the best. All fixed costs are cut in half but in practice it is pretty much like you have your own airplane as you will hardly ever have scheduling conflicts. Also airplanes like regular use and it really takes 2 owners to give the airplane enough exercise.
FYI for the 4 plane partnership we assigned "ownership" of the airplane on a weekly rotating basis . If you wanted to fly the aircraft when it was not your week you had to get approval from that weeks "owner".