I had a different, but related, issue in a datasource cleaning project. I had 1M+ PDFs in a dataset from a source that I didn’t have permission to edit, and after unsuccessfully trying to convert it to a media set, Palantir informed me that some of the media set functions had an undocumented limit of 10,000 PDFs.
For attachments, the limits are documented there.
As of August 2024: 200 MB, one attachment can be linked to 10 objects in its lifetime. Attachments not linked to objects are eventually automatically deleted.