Try this
SELECT t2.Bo_Title, COUNT(*)
FROM w3t_Posts as t1,
w3t_Boards as t2
WHERE t1.B_File <> 'NULL'
AND t1.B_Board = t2.Bo_Keyword
AND t1.B_Board = 'forum keyword' // <-- fill in keyword
GROUP BY t2.Bo_Title
Or this if you wish to show them all...
SELECT t2.Bo_Title, COUNT(*)
FROM w3t_Posts as t1,
w3t_Boards as t2
WHERE t1.B_File <> 'NULL'
AND t1.B_Board = t2.Bo_Keyword
GROUP BY t2.Bo_Title
ORDER BY t2.Bo_Title ASC