No - EndNote libraries are designed to be “unlimited.” There should be no limit on records, file attachments
[You can attach up to 45 file attachments per record in an library - but no limit to the total number of files attached to a whole library.], etc - beyond file size limits that the operating system will impose. “Unlimited” is admittedly a pretty hard number to test and verify but we do regularly test internally with EndNote libraries with about a quarter of a million records - many with file attachments. For files of this size, performance is going to depend on the operating system, RAM, and processor much more than EndNote itself.
Based on our on-going tests, and input from many customers, if you have an EndNote library with several thousand records - many with file attachments - on a relatively new and fast computer, you should have no issues.
If you are experiencing something different, please let me know and I will have the team look into it.
Jason Rollins, the EndNote team