OK I'm a stats nerd, so I ran some queries on our database for PokemonDb to find the answer. I'm sticking to Ultra Sun/Moon only. Note: our database is missing a few special moves, but they should not affect the average - we'd need to be missing 800 moves for the average to go up 1, and I don't think we're missing that many!
Excluding duplicate moves (e.g. a Pokemon learning the same move via level up and TM), we have 61,249 distinct Pokemon-move combinations across 843 Pokemon. Yes 843 Pokemon - those extra ones are due to alternate forms (we actually have around 100 alternate forms listed, but many of them such as Megas are not counted here).
So the average Pokemon can learn 73 moves! (or 72.656 to be more precise)
My original answer before I updated it was not counting transfer-only moves. I think it's also valid approach. You can call it the "purist method" - counting only the moves a Pokemon can directly learn in USUM. This would also only count breeding moves on the baby Pokemon (e.g. Bulbasaur learns Giga Drain by breeding, but Venusaur technically does not because Venusaur can't hatch from an egg).
Using this method, we have 48,692 distinct Pokemon-move combinations across 843 Pokemon. That gives us an average of 58 moves per Pokemon (or 57.760 to be more precise).