Answer to your question: 2.
All these data, sql and management nodes can run on a single machine. With two such 3-node machines you have full redundancy.
Keep in mind that NDB is in-memory. Failure of all nodes means not "service temporarily unavailable", but "all data is gone forever". I don't know how many nodes I need in such a setup to sleep at night. At least add some replication to disk database.
My personal conclusion was to not use MySQL cluster at all. There's a plenty of clustered databases today.
