As the natural join take the common attribute and combines according to them lets . take a relation R(a,b,c) an a relation S(a,p,q) such that no value of a of r is repeated in the attribute A of relation s. so natural join will take the cross product and will search where R.a=S.a so nothing will be selected and zero tupples will be returned, while if we does not have common attributes then it take the cross product an as it now does not which to compare as there is no common attribute it, stops there only providing you the cartesian product.
so minimum - zero
max- R*S.
and the min max will change according to join or the constraint