19 and 99:
Maybe this isn't very interesting, but when the members of the ka-tet were introducing themselves at the Calla party, they started to say 19 and 99 (book V), it seemed sort of random. I know 1999 is when the SK accident happened, but maybe there is something more to the 99. I see many of us agree that Roland is in the 19th loop. Thus, counting Oy, that is a total of 95 current and former ka-tet members. Then you can add Cuthbert, Alian, and Susan from book IV, along with Roland again, to get to 99. 99 total ka-tet members. I want to say Susan was an official card-carrying ka-tet member, but I'll know on my next read through of book IV (I'm going through again, on book III).
Not the deepest thought ever, and looking for a pattern where there may very well be none intended, but thought I'd just throw it out there. I don't think it helps me form a better understanding of the story per se, but is slightly interesting.