Prime... numbers...? What are these magical words that are coming out of this guy's mouth? To think that, in my years of learning the secrets of numbers (learning about the first hundred was tough...), I would actually encounter a different species of numbers... this manga is really something.
That aside... author just made use of prime numbers as a simple way to create an increasing gap between power levels. And add a hint to how power escalates.
I'm expecting all/most top class mages to have those rings and an extra... if suddenly the rings become a super valuable, super rare thing that only 7 people in the country are known to have, then I'd say the passing mentor guy is a very obvious plot device that is very unreasonably broken and needs a fix.