On-delete set NULL would set 3 in table T2 under S column as NULL. That's it. No record needs to be deleted off of T1.
Ans: 0
However, if On-delete-cascade was the situation, < 8, 3> would be deleted from T2, which would in turn result in the deletion of < 5, 8 > and < 9,8 > from T1.
Answer would have been 2 in this case.
PS: 'On delete set null' would make the row < 8,3 > as < 8, NULL> and not <NULL, NULL>