I used to write combat simulators for 1e on my C64 to figure out just what a fighter of a given level could be expected to handle.
I remember that for goblins or kobolds it was in the high hundreds. Even for 1st level fighters or orcs, it would have been more than 200 opponents assuming decent stats for the 10th level fighter and average ones for the 1st level fighters and no massed ranged weapons fire. I would expect that 3e plays out similarly, though with somewhat better odds for the monsters.
I personally feel that the number ought to be between 30 and 100, depending on the skill, tactics, and equipment of the 1st level fighters. Ranged weapons and reach weapons help the mooks quite a bit here, as would good armor. Obviously, if the mooks all have longbows and begin attacking at long range, the situation is much more favorable for the mooks than if they all have clubs and leather armor.
By 10th level, I feel that you are dealing with the storied figures of legend and song - Roland, Achilles, Cerano D'Bergerac, John Carter, etc.
In 4e terms, I guess 10th level = 15th level, so assume my answer is for 15th level if you are thinking on this from a 4e perspective.