Directory Server 6.3 does not limit the number of masters in a multi-master topology. A fully-meshed, multi-master topology with no hubs or consumers is recommended in most cases.
Advantages of an all-master topology include the following:
Availability. Write traffic is never disrupted if one of the servers goes down.
Simplicity. In an all-master topology, there is no need to set up referrals to route reads and writes to different servers.
There may be reasons that an all-master topology is not viable in a specific deployment. For example, fractional replication cannot be used in an all-master topology because fractional replication is only supported from masters to consumers.