There is no limit to the number of key vault however there is a limit of number of resources per resource type you can deploy within a resource group . This means , lets you have 800 applications within a single resource group and you create one key vault per application then you have created 800 instance of same type of resource (in our case Key vault) and you may not be able to deploy more than that within the same resource group . You will have to create a new resource group to deploy new apps and related key vaults.
The limit is based on resource types. Some resource types can exceed the 800 limit and are not limited to 800 instances per resource group. Azure Key vault has some Service limits and Throttling guidance which you may want to consider for large environments and for designing secure applications heavily relying on Key vault operations.
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