In a Consul cluster, participating nodes can be only one of two types.

Within each datacenter, we have a mixture of clients and servers. It is expected that there be between three to five servers. This strikes a balance between availability in the case of failure and performance, as consensus gets progressively slower as more machines are added. However, there is no limit to the number of clients, and they can easily scale into the thousands or tens of thousands.

Server or Leader - It indicates whether the agent is running in server or client mode. Server nodes participate in the consensus quorum, storing cluster state, and handling queries. At any given time, the peer set elects a single node to be the leader. The leader is responsible for ingesting new log entries, replicating to followers, and managing when an entry is considered committed.

Client or Follower - Client nodes make up the majority of the cluster, and they are very lightweight as they interface with the server nodes for most operations and maintain a very little state of their own.

Reference link:- https://www.consul.io/docs/internals/architecture.html