I do not agree with the current answer because it only gives the total size of the cluster.

The question is about node-pools. I actually needed to find out the size of a pool so I give you my best shot after many hours of searching and thinking.

read -p 'Cluster name: ' CLUSTER_NAME
read -p 'Pool name:    ' POOL_NAME

gcloud compute instance-groups list \
 | grep "^gke-$CLUSTER_NAME-$POOL_NAME" \
 | awk '{print $6}';

The gcloud command returns 6 columns: 1-name, 6-group-size. The name of the instance group is predictable which lets me filter by that line with grep. Lastly, select the 6th column.

Hope this helps someone else save some time.

For some reason I overlooked the not-so-obviuos from Migrating workloads to different machine types

kubectl get nodes -l cloud.google.com/gke-nodepool=$POOL_NAME -o=name \
  | wc -l