An Availability Set also makes use of Update Domains. This allow you to determine how many of the workloads are down at any given time. You can set a priority order for shutting down the VMs and the number of update domains determines how many machines will be involved in the shutdown. In the image below, we see an Availability Set with 16 virtual machines, and four update domains. This means that a maximum of four VMs can be down for maintenance at a given time, allowing the other 12 to carry the load. Once the first four return to service, another group will be available for maintenance. In conjunction with Fault Domains, this allows an Availability Set to ensure that undue burden is not placed on either rack.
when considering your use case, including the number of VMs you want to create and the number of Availability Sets you will need to create, remember that as a rule, you want one Availability Set per workload. A workload can be considered any virtual machines working together towards a common single purpose. Therefore, two highly available SoftNAS VMs to perform a single function would constitute a workload.
See Azure Availability Sets in the High Availability section to learn how to set up Azure Availability Sets.