Havent found a concrete answer, but what could be the greatest number of legs that you have done or have seen under 1 DL PNR?

I know there are 6 city pairs, and I have seen a max of 3 legs per city pair, so that would be 18 maximum legs, but is there an exception to the rule, say an IRROP, how would that be changed in the prn? a new tix ?

just wondering how the system works.