Pierre commented that in_use refers to VMMs (physical servers), not VMs. If you want the number of instances, either read the vms file, or do the same I am doing: parse the nimbus_admin.json file, select 'state' == 'Running' instances, and count them. Note that we could have more than 100% utilization because Nimbus allocates only with regards to memory, not CPU, which means we could have more virtual CPUs than physical ones.