This may be what your after:
SELECT Count(Owner_ID), Name
FROM (
SELECT M.Owner_ID, O.Name, T.Type
FROM Transport As T, Owner As O, Motorbike As M
WHERE T.Type = 'Motorbike'
AND O.Owner_ID = M.Owner_ID
AND T.Type_ID = M.Motorbike_ID
UNION ALL
SELECT C.Owner_ID, O.Name, T.Type
FROM Transport As T, Owner As O, Car As C
WHERE T.Type = 'Car'
AND O.Owner_ID = C.Owner_ID
AND T.Type_ID = C.Car_ID
)
GROUP BY Owner_ID