More detail would be good. What hardware and IOS are involved?
I'm assuming your question is about the depth of Q-in-Q tags, and not the number of tags presented by the customer's network.
This may or may not be applicable to you, but you can download the CSR1000v IOS-XE platform and run it in your VM environment for testing. If you don't have a VM host, you can download a free hypervisor, like VM Workstation Player, and run the router on your desktop.