Each thread used to run the EvaluateJsonPath processor will pull one FlowFile from an incoming queue and evaluate against one or more configured JsonPath expressions.
By default every processor you add to your canvas is only configured to use 1 concurrent task. You can add additional concurrent tasks to a processor. By doing so each concurrent task can pull a different file form an incoming queue and work on those files in parallel. Each Concurrent task works on its own FlowFile. Keep in mind that each running concurrent task is another cpu thread. So if you configure 10 concurrent tasks on a processor and their are enough incoming queued FlowFiles, this one processor could end up consuming 10 threads from your NiFi thread resource pool.
The "Max Timer Driven Thread Count" from which processor get their threads (concurrent tasks) is configured via the "controller settings" UI found in the upper right corner menu of the UI. This values is defaulted to 10. This controls how many system threads the NiFi controller will consume from your hardware to service thread requests form your various components added to the NiFi canvas. This value should be set between 2-4 times the number of cores you have available. If you are a cluster, this values is set per node and not per cluster. So if you have 4 nodes each with 16 cores, this values should still only be set to 32 to 64. Be sure to monitor CPU utilization on your server(s) to see what inpact these changes have on your hardware.