Three master nodes are enough, for as many data nodes as you wish to add. You can search this mailing list for discussions where kimchy explained the "dedicated master nodes", and how it fits for split-brain situations
For example https://groups.google.com/forum/#!topic/elasticsearch/dxjpMd4vNXQ Jörg -- You received this message because you are subscribed to the Google Groups "elasticsearch" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/elasticsearch/CAKdsXoGkxGgMCD_0cdSv9erR1qupr2ceghaa5rXFsEbNO_aadw%40mail.gmail.com. For more options, visit https://groups.google.com/groups/opt_out.
