mongodb副本级是需要三台服务器吗

至少3台：一台活跃节点，一台备份节点，一台选举节点。
30之前集群最多有12个节点，30之后的版本最多可以有50个节点，但 只有7个有选举权。
官网描述：
The minimum requirements for a replica set are: A primary, a secondary, and an arbiter
……
Changed in version 300: A replica set can have up to 50 members but only 7 voting membersIn previous versions, replica sets can have up to12 members
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