DECLARE @T TABLE
    (Col1 INT, Col2 INT, Col3 INT, Col4 INT
    ,Col5 INT, Col6 INT, Col7 INT, Col8 INT
    ,Col9 INT, Col10 INT, Col11 INT, Col12 INT)

INSERT INTO @T (Col1, Col2, Col3, Col4)
SELECT 1, 2, 3, 4
UNION ALL SELECT 5, 6, 7, 8

SELECT * FROM @T

SELECT TOP 1 n
FROM (SELECT TOP 1 * FROM @T) a
CROSS APPLY (
    VALUES (1, Col1), (2, Col2)
        ,(3, Col3), (4, Col4)
        ,(5, Col5), (6, Col6)
        ,(7, Col7), (8, Col8)
        ,(9, Col9), (10, Col10)
        ,(11, Col11), (12, Col12)) b(n, Col)
WHERE Col IS NULL
ORDER BY n

SELECT TOP 1
    CASE WHEN Col1  IS NULL THEN  1 WHEN Col2  IS NULL THEN  2 WHEN Col3  IS NULL THEN  3 WHEN Col4  IS NULL THEN  4
         WHEN Col5  IS NULL THEN  5 WHEN Col6  IS NULL THEN  6 WHEN Col7  IS NULL THEN  7 WHEN Col8  IS NULL THEN  8
         WHEN Col9  IS NULL THEN  9 WHEN Col10 IS NULL THEN 10 WHEN Col11 IS NULL THEN 11 WHEN Col12 IS NULL THEN 12 END
FROM @T