It's not scaleable. Couchbase Server does not scale to large numbers of buckets, mostly because each bucket is pre-allocated a fairly large amount of RAM. It's not recommended to have more than a dozen or two buckets in a cluster.
You should try to design your system to use a single database shared by all users. The Sync Gateway was expressly designed to support this, so it has a lot more access control than CouchDB does.
Back in 2012 we tried to build a system (SyncPoint) based on CouchDB that used a one-db-per-user model, and realized it wasn't going to work well. It was kludgy to configure and not scaleable, especially if users shared documents.
—Jens