ODL clustering has no notion of discovery. There is a flat file (akka.conf) on each VM that contains info about its peers.
Basically the two main limitations are (a) the total number of nodes has to be odd (due to how voting works) and (b) if you add additional nodes, you need to update the akka.conf on all the existing nodes as well and bounce them. Make sure to bring up the old ones first to make sure one of those becomes the "shard leader" (otherwise I'd worry that one of the new ones would think it's the leader even though it has no data yet, and "replicate" its lack of data to the others).
ODL clustering was designed for high availability really - not for scaling. So that's why it doesn’t address scaling in any real way. There are some improvements in clustering planned for the next ODL release (Oxygen), the details for the improvements are not yet known.