I really need to stop thinking in these terms
I've run the same campaign world using different rules, so that has also provided for me a distinction from one campaign to another.
As far as the text in the OD&D rules, I envision a scenario akin to what grodog posted, that there was often a co-DM in Kuntz. Clearly the "from four to fifty players" bit is the number of people adventuring in said campaign world, as noted by Falconer.
My largest single sitting sessions probably topped out at 8 or 9 back in the early 80's. I know certain players prefer small parties, while others can flourish in large or small parties. I prefer 5 players, I'll play with as few as 2, and I wouldn't be opposed to playing with 10-12 provided the players were OK with much longer intervals between actions.