I personally have used CouchDb to store tens of millions of documents. If you can find a way get the data you want using CouchDb views, the number of documents

I personally have used CouchDb to store tens of millions of documents. If you can find a way get the data you want using CouchDb views, the number of documents simply doesn’t matter with CouchDb (may be just the disc usage grows with additional documents/views). And that too with excellent performance.