Near the top of the NiFi screen under the Component toolbar is a bar that is referred to as the Status Bar. It contains a few important statistics about the current health of NiFi. The number of Active Threads can indicate how hard NiFi is currently working, and the Queued stat indicates how many FlowFiles are currently queued across the entire flow, as well as the total size of those FlowFiles.
If the NiFi instance is in a cluster, we will also see an indicator here telling us how many nodes are in the cluster and how many are currently connected. In this case, the number of active threads and the queue size are indicative of all the sum of all nodes that are currently connected.