When I think about it, I've seen them all at least once.
But if you mean less than a handful of times or extremely rare, surprisingly there's a quite a few of them.
It's weird. As an example... days consecutively, I'll get mostly ZSS or Lucario the entire night, even when it has nothing to do with Conquest. Then when a match goes bad with 1 character and I should probably be banned, I get paired up with the same character the following match. It's like Nintendo has people playing you waiting to see if your connection is good or if you don't like a certain character.
Weird ****.
It's different on a week per week basis. I don't know what's going on behind the scenes with this game really.