You should use 1 app per network. For example, if my hand was my VLAN, fingers were my endpoints, arm my connection line to the internet then I'd place the app on my arm behind any mitigations going outbound. 

All traffic will pass through that line therefore, you can test network mitigations using 1 app. However, if you'd like to test a specific endpoint it would require 1 license per endpoint. 

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