Out of about 15 characters, only 1 has multiclassed (monk/rogue).
There is also a bard that might take a level of wizard at higher levels for concept purposes (and I actually talked the player into that, because they wanted me to allow some custom bard content I wasn't comfortable with, and this solution is better.) And there is one character who is my custom Warrior-Mage class, because none of the multiclassing (or single class) combinations really let me represent my vision of an AD&D fighter/wizard.
So "hardly any" is the answer from my table, which suits me just fine. I really like that characters are sticking with class identity (classes have actual in-world identity in my setting) rather than trying to build their own class by multiclassing (which seemed really common in 3e).