When the Captain determines the number of men attacking from each side, why doesn't he change his strategy?

The Captain doesn't change his strategy because he knew if the pirates crossed the boundary of the stockade, they'd all be dead anyway.

But this was not so easily answered. There had come many from the north—seven by the squire's computation, eight or nine according to Gray. From the east and west only a single shot had been fired. It was plain, therefore, that the attack would be developed from the north and that on the other three sides we were only to be annoyed by a show of hostilities. But Captain Smollett made no change in his arrangements. If the mutineers succeeded in crossing the stockade, he argued, they would take possession of any unprotected loophole and shoot us down like rats in our own stronghold.