If you don't want to choose an AZ at cluster creation or are worried about IP availability you can use the Automatic Availability Zone (Auto-AZ) feature. This will configure the cluster to automatically choose an AZ when the cluster starts based on the availability of IP's in the workspace subnets.
Currently this is only available through the API. But it can also be enforced with cluster policies, which would apply to clusters created through the UI if that is a requirement.
You can see more about the feature here in the documentation.
