Is there a limit for the number of nodes in an indexer cluster, above wich we will see a performance degradation? With our current growth rate we will probably come towards 128 indexers, each with 16 PB local storage, in a two site indexer cluster. There will also be a search head cluster with 20 to 30 search heads in tow different sites.
from a clustering perspective, as the number of buckets approaches 1 million + we start hitting some performance issues.
having said that, we've scaled to 1k+ indexers, and several million buckets+ recently with Splunk 6.6. there are some recommended timeouts we advocate raising for larger clusters (support should be able to help you out with that!)