SELECT COUNT(*) AS Count FROM w3t_Subscribe WHERE S_Board='boardname'
That would give you a number of how many times your 'boardname' was referenced in the Subscription Table.
You could do the same thing to figure out how many boards a particular user subscribed to (if you plugged in user #537, you had 33 boards, and the Count came back 22, he is subscribed to 22 boards).
Have fun.