Our docs give the wrong advice that you should have an odd number of voting members in a replica set: https://docs.mongodb.com/manual/core/replica-set-architectures/#deploy-an-odd-number-of-members

There is nothing in the replication protocol or the election algorithm that requires an odd number of voters. Many customers however think that there is such a requirement, and the passage at the above link may reinforce that misunderstanding.

As the page explains one section down, an even number of members does not buy you any additional fault tolerance compared to the next lower odd number, which is why an odd number of members makes sense from an economical standpoint, but there is no technical requirement for this. I suggest removing the first paragraph at the above link entirely, and giving the section a different heading.