Coalition membership used to be unlimited (iirc) then it was reduced to 5, eventually the current system was emplaced, 3 for Small Maps, 4 for Medium Maps and 5 for Large Maps. There was and still is good reasons for these numbers, unless you can provide good reasons to increase these numbers other than because, they should stand as is.
"Strategy is the art of making use of time and space. I am less concerned about the later than the former. Space we can recover, lost time never." ~ Napoleon Bonaparte
"Anyone who has to fight, even with the most modern weapons, against an enemy in complete command of the air, fights like a savage against modern European troops, under the same handicaps and with the same chances of success." ~ Erwin Rommel