IBM's "Cloudant" is not a web app per se, it's db as a service, but the service is a db per user.
You might want to check out the CouchDB docs, and maybe take a look at their mailing list for both users and developers. Their dev team can provide the answers you're looking for as far a CouchDB goes.
It's my understanding that scaling up or down is a key feature of CouchDB. It's designed to make it easy to create clusters of them working together. But I really do not know much about that myself.
You might want to check out the CouchDB docs, and maybe take a look at their mailing list for both users and developers. Their dev team can provide the answers you're looking for as far a CouchDB goes.
It's my understanding that scaling up or down is a key feature of CouchDB. It's designed to make it easy to create clusters of them working together. But I really do not know much about that myself.