how to select the most frequently appearing values?

I've seen examples where the query orders by count and takes the top row, but in this case there can be multiple "most frequent" values, so I might want to return more than just a single result.

In this case I want to find the most frequently appearing last names in a users table, here's what I have so far:

select last_name from users group by last_name having max(count(*));

Unfortunately with this query I get an error that my max function is nested too deeply.