Unicorn is for web requests. Running 5 means you can handle 5 “slow” web requests concurrently. Certain web requests like “message bus” and “avatar caching” and “uploads” run in background threads, so the real number of concurrent requests tends to be a lot higher.
“concurrent users” highly depends on what the users are doing and logged in vs anon which is heavily cached. I can’t really provide a particular guideline, but I can provide instrumentation that will tell you if there is a problem.