3249.
(You can check by seeing the numbers next to the load more button on the all-posts page for 2021.)
Huh, this was approximately the number I'd have guessed, but I remembered you should probably filter out events and low-karma posts, and I'm actually fairly surprised that when you do that the number drops down to 1040:
https://www.lesswrong.com/allPosts?after=2021-01-01&before=2022-01-01&limit=100&timeframe=yearly&includeEvents=false&karmaThreshold=0
Woop! Pretty good results. A few of my +9s aren't in the top 50, but most of them are. And well done to Elephant Seal 2, ranking higher than Elephant Seal 1 did.
Man I find myself curious about whoever medium-downvoted "The Death of Behavioral Economics". This seems like it throws a wrench in some of the original underpinnings of LessWrong. I get not thinking it was all that important, but surprised someone would vote strongly against it.
Presumably they agreed with Scott's criticisms of it, and thought they were severe enough problems to make it not Review-worthy?
I didn't get around to (?re-)reading & voting on it, but I might've wound up downvoting if I did. It does hit a pet peeve of mine, where people act as if 'bad discourse is okay if it's from a critic'.
Results
Voting is visualized here with dots of varying sizes (roughly indicating that a user thought a post was "good" "important", or "extremely important"). Green dots indicate positive votes. Red indicate negative votes. You can hover over a dot to see its exact score.