We have many projects with lots of documents and many being very large. Although most are not assembled documents and many have lots of versions.
We see the biggest impact on creating PDFs from multiple documents, in that the time it takes to create the assembled PDF is directly proportionate to the amount and size of the documents being assembled.
And the page loads would be slower when accessing the document tab in teh full view or the overview in the compact view. Though you might be able to reduce this if you use folders... then of course you'll have to deal with the navigation in the folders, though conceivably you could use some quick links to access various places in the folder structure.
I don't think we have anything in the thousands, at least not visibly, maybe due to versions and such. But as yet we haven't had any complaints of anyone being unable to add a document because there are too many in the workspace.
I would expect that if there were a real limit there would be a parameter setting that could regulate the number of documents in a workspace.
Unfortunately we don't make heavy use of the ACM projects but I wouldn't expect to see a big difference, however the types of documents that can be housed in each is different. As far as I know the ACM projects cannot house assembled contract documents such as the main agreements and/or contract addendum.