Most horrifying is a 2014 Science paper on Ebola with 58 authors, 5 of whom died of Ebola before publication. https://www.science.org/content/article/ebolas-heavy-toll-st...

The ATLAS-CMS joint Higgs boson mass measurement paper has close to 6000 authors: https://doi.org/10.1103/PhysRevLett.114.191803

I think the final COVID consortium report has something like 30k authors.

As another comment points out: that's nothing compared to high energy/particle physics!

The Pierre Auger Observatory certainly is a large collaboration for the astroparticle physics domain though. It's a big international collaboration.

Quick anecdote: my name (S Mueller) is not on that paper's author list because we had a rule that you had to be in the collaboration for a year before getting authorship. You stayed on for a year after leaving. Very reasonable! At the time I was nonetheless a bit bummed about missing out on the big Science paper. I guess I'm on the retraction though ;)