If you have phpMyAdmin, a real easy way would be to go into the post table and find all posts by that user. It then tells you in the query how many rows get returned. I think if you just want to make the query, it would be something like:
Code:
SELECT count(*)
FROM `post`
WHERE `userid` =x
where x is the userid. (That does not take into account table prefixes. It's copied straight from phpMyAdmin - except I put count(*) in the select part of the statement.)