Those queries above will do exactly what you asked for. You say they dont what is the error you get. You can also suppliment the WHERE U_Username = 'USERNAMEHERE' to WHERE U_Number = 'usernumberhere' Make sure you are not forgetting the slashes around the name or number in the where clause.
As far as getting a post count
SELECT COUNT(*) FROM w3t_Posts WHERE B_PosterId ='USERNUMBERHERE'
Then to update the users total
UPDATE w3t_Users SET U_TotalPosts = 'TOTALHERE' WHERE U_Number = 'USERNUMBERHERE'
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