I am a KNIME Small Server user and I believe the answers to your questions are probably the same for KNIME Server large as with Small.
RE: In this situation, how should I interpret “maximum users = 5”
I believe this is the number of logins you get for the webportal/server (including the admin user). So 5 individuals can login and deploy workflows or check webportal activity. It’s nothing to do with the OS.
I believe 5 people should be able to access the webportal simultaneously. Certainly 5 should be able to log in to the server via analytics platform.
Unless you have a massive organisation, I would not worry too much about the user limits. I have rarely had an issue with it. People who do have a log in can deploy workflows on behalf of those who don’t. The biggest disadvantage is that the workflow designer will not be able to monitor the workflows as they run or deploy them themselves. But they CAN be sent emails/alerts when the workflow fails or completes, there’s no restrictions on this.