The field is called replicas and you set it as part of the Deployment spec (not part of the Pod spec in the template):
apiVersion: apps/v1
kind: Deployment
metadata:
name: simple-web
spec:
replicas: 2
selector:
# ...
template:
# ...
The sample file in labs/aks/lab/deployment.yaml sets the replica count to 4.
You can use that file to update the Deployment, then if you watch the Pods you'll see three more get created:
kubectl apply -f labs/aks/lab/deployment.yaml
kubectl get pods --watch
When all the Pods are running, you can print the list with the -o wide flag to see which node the Pods use:
kubectl get pods -o wide
You should see Pods spread across your two nodes.
Browse back to your public IP address - the Service IP address hasn't changed. Refresh without using the browser cache (e.g. Ctrl-F5 on Windows) and you'll see response coming from different Pods. The Pods all have the app=simple-web label, and the Service load-balances requests between them.