Instance mix in Azure Virtual Machine Scale Sets allows you to define multiple VM sizes within a single scale set that uses Flexible Orchestration Mode so Azure can choose among them during provisioning. This capability improves provisioning success, helps reduce cost, and supports predictable allocation ordering when your workload can run on different VM sizes

What instance mix does

• Lets a single scale set use more than one VM size (up to a defined limit, typically five sizes) instead of a single fixed size.
• Works only with scale sets configured in Flexible orchestration mode, not Uniform mode.

How it works conceptually

• You configure a list of allowed VM sizes in the scale set configuration and choose an allocation strategy (for example, lowestPrice, capacityOptimized, or a prioritized option).
• During scale-out, Azure evaluates capacity and, depending on the chosen strategy, selects from your allowed sizes to meet the desired instance count while honoring regional capacity and your priorities.

Prerequisites

• A scale set configured to use Flexible Orchestration Mode; instance mix is not available for Uniform mode.
• Instance mix must be supported in the selected region when you create the scale set.
• Sufficient vCPU and VM size quota in the target subscription and region for each VM size included in the instance mix.
• A virtual network and subnet available in the target resource group when deploying via templates, CLI, or REST.
• All selected VM sizes must share consistent architecture (x64 or Arm64), storage interface (for example, all SCSI or all NVMe), local disk configuration, and security profile.
• Only supported VM families (A, B, D, E, F) can be used in the skuProfile.vmSizes list, and you can specify up to five sizes.

Example configuration concept

An instance mix example is a Flexible VM scale set that can use several D‑series sizes, letting Azure pick among them based on price or capacity. Below is a concise conceptual example using three sizes.

• Orchestration mode: Flexible (required for instance mix)
• VM sizes in the mix (all same architecture/family)
  • Standard_D2s_v5
  • Standard_D4s_v5
  • Standard_D8s_v5
• Target capacity: 10 instances, allocation strategy set to lowestPrice.​
• Azure attempts to allocate the cheapest size first (for example, D2s_v5), and if capacity is constrained, it fills remaining instances with D4s_v5 and D8s_v5 according to available capacity and your configured priorities.

Reference: Instance mix can be used with multiple azure virtual machine sizes

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