After researching and testing a long time,,, regardless of whether there is a maximum NUMBER of files to attach to a OneNote workbook, ultimately, if you are attaching a large number of files then this is not the best practice for a number of reasons I've found...
1) Cache can break
2) Adding large number of files creates a lot of variables in the loading and backing up of the file and can lead to crashes.
3) It's not the best practice for file management generally, but especially if you are storing the workbook in a replicated folder (i.e., DFS), or folder that has any type of versioning applied to it (like Windows Shadow Copies).
If you're attaching a large number of files then it would be better to use a file management solution (i.e., Sharepoint), or with standardized file/folder structures with linking of files in OneNote instead of attaching them.
For law firms, I would specifically recommend CaseMap.
This doesn't specifically answer my own question, but it's the conclusions I've come to.