Index with a large number of document types?

My app needs user based searching, most users will have a small number of documents, so I think index-per-user design is probably not a good idea. I am considering using document type per user architecture. For example:

documents/document-user1
documents/document-user2
documents/document-user3

The number of users can potentially be large, is there any performance problems or disadvantages for an index with hundreds of thousands of mapping types ?

Thanks,
Peng