I think this should work:
Code:
SELECT username, COUNT(*) as count FROM prefix_post
WHERE dateline BETWEEN UNIX_TIMESTAMP(YYYYMMDD) AND UNIX_TIMESTAMP(YYYYMMDD)
AND visible = 1
GROUP BY userid
ORDER BY count DESC
(of course you have to insert your own table prefix, and the dates you want in place of the YYYYMMDD)
ETA: ...and you probably need "AND visible = 1" in there to eliminate deleted posts (so I went ahead and added it).