Please refer to this doc on metrics for autoscaling VMSS:https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-autoscale-overview#autoscale-rule-criteria
You can also autoscale using custom metrics as described in this document: https://learn.microsoft.com/en-us/azure/azure-monitor/platform/autoscale-custom-metric