Sorry about the Contact Sales bit; it's temporary. Our full intention is to allow self-serve purchasing of these things, it's on our near-term roadmap.
In your current plan, all devices are grouped into a single category, and you are allotted 100 devices plus an additional 20 devices per user in the tailnet. We've typically been pretty generous on enforcing this limitation. In the new plan, we have instead offered unlimited devices owned by users, and 50 owned by tags.
I'd love to understand more about your use case and understand how we can make the transition easier for you (not a sales call). Feel free to reach out sam [at] tailscale.com if you'd like to chat!