The vBuckets(bucket) really are limited by your server size. Couchbase writes all your data to Hard Disk and just puts the Most Recently Used Items into memory.
Sounds like you are moving from RDBMS to Key=>Value(Documents).
Its best if you read over document Design.
http://www.couchbase.com/docs/couchbase-devguide-2.1.0/relating-documents-for-retrieval.html
It will give you insight into how to put documents into couchbase
Tool to Move your RDBMS to Couchbase
http://blog.couchbase.com/mysql-couchbase-using-talend-etl