Those are pretty respectable numbers! I recall making a point of writing all the intra-Nsync pairings and all the Nick/Nsyncer crossover pairings, but hadn't set out to cover the rest. Still, I find I've written ten crossover pairings (including the Nick ones) and three intra-Backstreet. And if the counting is more generous, my fics do mention one other crossover and four other Backstreet pairings having happened in the past, plus the gray area of the Joey/Lance/AJ threesome in which Joey and AJ each only touch Lance. So that puts me at twenty-three pairings written, six underserved, and sixteen entirely untackled. Hmmm.