Raft leader node sends append entries RPC to all followers. Obviously we increase network usage, when we add new follower, so my question is about how much nodes we can add to cluster. In Raft paper and in other places I read that 5 nodes in cluster is optimal choice, but what you can say if we will have 100 nodes in cluster?
Yes I understand that I can calculate limit, will be enough network bandwidth or not. My question is more general, is cluster with tens of nodes sign of bad architecture?
Yes, a cluster with tens of nodes is generally a bad idea. Typically, we see clusters go up to 7 nodes, but not really beyond that, and even that's atypical. 3 or 5 nodes is the most common.
If you want to scale across more than 3/5/7 nodes you typically just shard the cluster, where each shard runs a completely separate and independent instance of the Raft protocol. If you need to scale for fault tolerance, you will have to relax consistency requirements.