Using the OAD&D rules, a Dm could handle 20 players withoug much difficulty. As a matter of fact I was the lone DM at a DMG-sponsored con, and I ran five groups of 20 through a torurnament adventure.
Remember PCS were not expected to be long-loved back in those days
I admit i was a bit sanguine when I mentioned 50 as the upper number. Anything over 20 and I called fro a co-DM (referee) to assist in keeping track of who was doing what--and rolling the dice for the hordes of monsters such hige parties attracted.
The more "refined" game sessions where there was actual roleplay, problem solving, and the rest were always done with much smaller groups, usually one to four or five players,