A single Evernote account can't accommodate unlimited no. of notes, notebooks, tags. So, I'm worried about it. What happens after reaching the limit? Do they say to create new account or close account? Is there any workaround or prioritized action for premium customer? Has anyone run into this problem before?
The limits are considerably generous and practically unreachable. I do not think that any one human being in a single lifetime will be actually able to exhaust these limits - not unless you are deliberately taking it as a sport
To answer your question, when the limit is crossed, you will not be able to create any more notes or notebooks or tags. Whether they ask you to delete a few things or create a new account is only a matter of their user interface and are only suggestions ad do not reflect on anything of technical nature. This is the same as reaching the total space limit in email providers, online storage and stuff like that.