That's wise to look ahead. No, I don't have an intuition for the theoretical max, but I'm sure we could tip over a zrok frontend if we tried hard enough.
If you can predict which dimension you're most likely to grow along then it's simpler to predict the point of failure, e.g., circuit saturation vs. CPU.
Have you already figured out how you're going to chart resource usage? zrok and Ziti have some great metrics and, combined with a system level monitoring, those will allow you isolate and analyze resource usage.
The solution for scaling zrok frontend is definitely horizontal replicas not vertical. Even a DNS round robin with a number of frontends will get you very far indeed.
Here's an informed perspective on performance: High availability architecture for routers and controller - #8 by mike.gorman
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