You want an uneven number of master eligible nodes, so you can form a proper quorum (two out of three, three out of five). The total number of nodes doesn't actually matter. Smaller installation often make the same node master eligible and data holding at the same time, so in that case you'd prefer an uneven number of nodes. Once you move to a setup with dedicated master eligible nodes, you're freed from that restriction. You could also run 4 nodes with a quorum of 3, but that will make the cluster unavailable if any two nodes die. The worst setup is 2 nodes since you can only safely run with a quorum of 2, so if a node dies you're unavailable.
Can you explain this one in more detail from a technical point of view?