How do you tell what hardware is a good fit at a certain level?

I’ve been evaluating Network hardware for the last few weeks. How do I figure out whether a given device will handle a certain use case? I’ve heard of different metrics: - X number of users - “10,000 to 20,000 concurrent connections” - 1,000,000 packets per second - 2.5Gbps or 1.5Gbps with firewall or 200Mbps over VPN

But the spec sheets don’t consistently address the same metrics.

Then I get anecdotal advice like “Brand X won’t handle 200 users. You need Brand Y.” Sometimes I can detect a flame war comment and sometimes it sounds legit. How can I objectively evaluate what I need and what hardware will fit?