Box 1: 2 -
Use two fault domains.
2 or 3 is max, depending on which region you are in.
Box 2: 20 -
Use 20 for platformUpdateDomainCount
Increasing the update domain (platformUpdateDomainCount) helps with capacity and availability planning when the platform reboots nodes. A higher number for the pool (20 is max) means that fewer of their nodes in any given availability set would be rebooted at once.
Reference:
https://www.itprotoday.com/microsoft-azure/check-if-azure-region-supports-2-or-3-fault-domains-managed-disks
https://github.com/Azure/acs-engine/issues/1030
