Using two partitions per table, which is best: even or odd numbers of nodes?

You just need 3 to be able to have H.A.(high availability). 

Also the K-Safety level that you need 1 or 2 will dictate the number of nodes you need. 

An Even number of nodes might get the split-brain phenomenon where half the nodes go down and your cluster is unable to determine which of the nodes remaining should be the control nodes, even with this if you always have available copies of data on the remaining nodes your databas should be ok.

1,5,6,8,13,21 nodes are just numbers ! You need to have the copies of data in order for your cluster to stay online(consistent).

Great explanation. Odd numbers it is.

There's no concern about the "split-brain phenomenom" for an even number of nodes.   Up to half the nodes can go down.  Or in other words, you need half + 1 nodes up for the cluster to remain up, so that the split-brain phenomenom is not an issue.

From the documentation, on k-safety:

"Note: If half or more of the nodes in the database cluster fail, the database will automatically shut down even if all of the data in the database is technically available from replicas. This behavior prevents issues due to network partitioning."

  --Sharon