This adventure is designed for 15-20 0-level characters or 8-10 1st-level characters. Remember that players should have 2-3 characters each, so they can continue enjoying the fun of play even if some of their PCs die off. In playtest groups of 15 0-level PCs, 7 or 8 typically survive. The author has playtested this adventure with groups of up to 28 PCs and experienced one complete TPK and several sessions with only a handful of survivors.20 characters? Think about the last time you had 20 people in your home and how crowded that felt. Were there even 20 people there, or did it just feel like that many?
Here is the dungeon map from the Basic D&D "Haunted Keep" that came in the old school rulebook. I've added 20 adventurers to the map and tried to place them in a realistic and organic manner as they move in from the entrance and explore.
Twenty is clearly *much* too many for a dungeon with 5 foot wide corridors (that's wider than the ones in your house) and rooms anywhere close to the scale. You'll note that I'm not using the "keep out of my 5 foot square" convention of later editions - but there's still not enough space for a platoon of adventurers to stay close together when moving around this map. A map that already greatly exaggerates the scale of a medieval style gatehouse!
I'm usually sympathetic to wanting a "dangerous" (aka high potential for character death) game, but after looking at the numbers and the dungeon scale I just don't think the answer is sending along this many backup characters in case one of the others gets killed. With this many adventurers you've essentially split the party by design - you're just making it a short distance between each of the sub-groups.
I could see 20 adventurers on a boat, in the wilderness, or on a battlefield -- but this just doesn't seem right for dungeon adventuring to me. So much so that I'm left wondering if one character per player, and a modest sized group (which I think is more common these days) actually IS the best approach after all.