Oh, you could definitely change that to "after X fishing attempts" where X is the number of PCs. I playtested the adventure with 4--5 people and did some solo testing, but didn't check larger group sizes.
The big advantage of a large group size is that it gives you more freedom to absorb exhaustion, tag in, use the right fisher for the job, etc, but this is counterbalanced by some fish and animals (and specifically the Great Dreamer Challu) scaling off of the party size. A large enough group might make Challu very very difficult without everyone getting more time to fish.
Good catch!