Reliability: A single ZooKeeper server (standalone) is essentially a coordinator with no reliability (a single serving node failure brings down the ZK service). A 3 server ensemble (you need to jump to 3 and not 2 because ZK works based on simple majority voting) allows for a single server to fail and the service will still be available. So if you want reliability go with at least 3. We typically recommend having 5 servers in "online" production serving environments. This allows you to take 1 server out of service (say planned maintenance) and still be able to sustain an unexpected outage of one of the remaining servers w/o interruption of the service.
感谢@sbridges指出它与维护法定人数有关。而ZK定义法定人数的方式是ceil(N / 2)，其中N是合奏中的原始数字(而不仅仅是当前可用的集合)。
现在，谷歌搜索ZK法定人数可以在HBase书chapter on ZK中找到：
In ZooKeeper, an even number of peers is supported, but it is normally
not used because an even sized ensemble requires, proportionally, more
peers to form a quorum than an odd sized ensemble requires. For
example, an ensemble with 4 peers requires 3 to form a quorum, while
an ensemble with 5 also requires 3 to form a quorum. Thus, an ensemble
of 5 allows 2 peers to fail and still maintain quorum, and thus is more
fault tolerant than the ensemble of 4, which allows only 1 down peer.
而维基百科的这个释义在Edward J. Yoon的blog：
Ordinarily, this is a majority of the people expected to be there,
although many bodies may have a lower or higher quorum.