Task: Count the items returned by a node

n8n workflow JSON:
{"nodes":[{"id":"41e0d0a9-9bd4-4ece-a204-5e1bf507b0eb","meta":{"instanceId":"cb9c144f2050b3f9b30bf379399398f9061341e3665eb2faf2b1092a42b38b14"},"name":"When clicking \"Execute Workflow\"","type":"n8n-nodes-base.manualTrigger","position":[820,400],"parameters":{},"typeVersion":1},{"id":"aa373efa-d493-44cd-91ee-e07630309675","name":"Customer Datastore (n8n training)","type":"n8n-nodes-base.n8nTrainingCustomerDatastore","position":[1040,400],"parameters":{"operation":"getAllPeople"},"typeVersion":1},{"id":"29555ae0-ad6c-4888-8865-c1e097b3b44e","name":"Set","type":"n8n-nodes-base.set","position":[1260,400],"parameters":{"values":{"number":[{"name":"itemCount","value":"={{ $input.all().length }}"}]},"options":{},"keepOnlySet":true},"executeOnce":true,"typeVersion":1}],"connections":{"When clicking \"Execute Workflow\"":{"main":[[{"node":"Customer Datastore (n8n training)","type":"main","index":0}]]},"Customer Datastore (n8n training)":{"main":[[{"node":"Set","type":"main","index":0}]]}}}