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Tournament modules had nine players normally. Home campaigns I played in usually had five or six players plus the DM but lots of henchman, followers, and hirelings as well which the tournament modules didn't normally have. So roughly nine competent PCs or six PCs and about six other NPCs maybe as a rough guide.
It's been years, but I'm fairly certain the G1-3 series was for 6-10 characters... it usually said on the front page or in the introductory paragraphs.
AD&D generally assumed a fairly large party size (usually about 6-8 PCs, possibly with henchmen/hirelings as well) by 3.x or Pathfinder standards. Of course, it was harder to "fill all the roles" with smaller parties, because of the way multi-classing (and "dual-classing") worked, along with the sparse skill (non-weapon proficiency) system.
For the most part, you actually needed about six characters (PCs and NPCs) to adequately "cover all the bases" for combat, healing, magic, locks and traps, and other functions (i.e., a ranger for tracking) with sufficient redundancy/staying power. Especially if you were using a stricter ability score generation method (i.e., the "default" was roll 3d6 in order); it was hard to qualify for some of the "better" classes, so you saw a lot of basic clerics, fighters, magic-users, and thieves (usually with fairly mediocre ability scores).
Modules would say in the beginning how many players and what level ranges but they were all over the place on how many players. 4-6 seemed normal. D&D itself just assumed however many were around of whatever level and you would go with mixed level and xp parties and varying party sizes. Party mix was assumed to be a factor in how you chose to approach things in the sandbox.