These factors can affect scale limit for host pools:
The Azure template is limited to 800 objects. To learn more, see Azure subscription and service limits, quotas, and constraints. Each VM also creates about six objects, so that means you can create around 132 VMs each time you run the template.
There are restrictions on how many vCPUs you can create per region and per subscription. For example, if you have an Enterprise Agreement subscription, by default you can create 350 vCPUs. You need to divide 350 by either the default number of vCPUs per VM or your own vCPU limit to determine how many VMs you can create each time you run the template. Learn more at Virtual Machines limits - Azure Resource Manager and Check vCPU quotas.
The VM prefix name can't exceed 11 characters, so that when a sequential number is added the total name is a maximum of 15 characters. To learn more, see Naming rules and restrictions for Azure resources.