Ok so I can count 515 touhou pairings on here, of which 15 are either triplets, Marisa x Everyone or nonsense like Zun x Beer or Rinnosuke x Anyone. Might be off by a few, but anyway, ~500.
The latest 2021 popularity poll (I'm not doing a touhou census myself) counts 206 characters, but at a glance the bottom 50 at least is troll picks (with all due respect to "hobgoblin" and "salt merchant" I'm sure they're cool guys). So let's round it down to 150.
Self-cest included, that makes 150x150 = 22.500 possible pairings, of which this site has only covered 500 so far, or 2.222%.
Weak.