I didn't count it, but I think papers from high energy particle physics have it beat. Some have over 5k authors.

Wow! You weren't kidding. People in my field often joke that papers in biology, health etc. tend to list every person they met on the day of submission as authors. But this one has 450 authors from 132 institutions. Surely this points to some sort of breakdown in the attribution system.

Tenure committees have to write a report detailing every single one of your papers and what your contribution was.