You deployed an application to Azure Availability Set with 2 VMs. You need to ensure that during planned maintenance of hardware, the application will remain available. Will fault domains or update domains help you? And how many of them do you need?

You need two Update domains that will make availability of your app happen during planned maintenance.

Fault domains define the group of virtual machines that share a common power source and network switch.

By default, the virtual machines configured within your availability set are separated across up to three fault domains for Resource Manager deployments (two fault domains for Classic). While placing your virtual machines into an availability set does not protect your application from operating system or application-specific failures, it does limit the impact of potential physical hardware failures, network outages, or power interruptions.

By default, you have 5 non configurable Update Domains by default (Resource Manager deployments can then be increased to provide up to 20 update domains) to indicate groups of virtual machines and underlying physical hardware that can be rebooted at the same time. When more than five virtual machines are configured within a single availability set, the sixth virtual machine is placed into the same update domain as the first virtual machine, the seventh in the same update domain as the second virtual machine, and so on. The order of update domains being rebooted may not proceed sequentially during planned maintenance, but only one update domain is rebooted at a time.