a crowd of
20 adventurers, if they're sitting quietly at camp, have a near perfect chance to hear anything mortal sneak up on them...but the bigger the search crowd, the more noise the party makes, which should increase the DC by some. I don't know by how much, but ever try listening in on something quiet when there'a a bunch of other blips and bloops and things going bump in the night ? (probably the dwarf in platemail, for instance).
Everyone gets their checks, but DC goes up by 1 per party member above, say, 3. Or everyone gets disadvantage from the start if you're more than 3. Less than 3, you're likely to be a swat team-style advance scouting sub-party anyway.