Extract rows of a table, that include less than x NULLs
Permissions to create a table in the current database shouldn't preclude you from creating one you can work with. You can just create a #temp table:

CREATE TABLE #test123(
    c1 float,
    c2 float,
    c3 float,
    c4 float,
    c5 float
);

INSERT #test123(c1,c2,c3,c4,c5);
VALUES (2,3,NULL,1,2),
       (2,NULL,NULL,1,2),
       (2,3,NULL,NULL,2),
       (NULL,3,NULL,1,NULL),
       (2,3,NULL,1,2);


To see why ISNULL isn't effective here, run this query:

SELECT ISNULL(c1,1), ISNULL(c2,1), ISNULL(c3,1), ISNULL(c4,1), ISNULL(c5,1)
  FROM #test123;


You've given every column in every row a value. So now you're evaluating the SUM of inflated values, and erroneously evaluating a property of the actual value (what happens when one of the values is negative?), instead of evaluating the COUNT of values that either are NULL or are NOT NULL.

It's more code but a simple way to address this is:

SELECT * FROM #test123
  WHERE CASE WHEN c1 IS NULL THEN 1 ELSE 0 END
      + CASE WHEN c2 IS NULL THEN 1 ELSE 0 END
      + CASE WHEN c3 IS NULL THEN 1 ELSE 0 END
      + CASE WHEN c4 IS NULL THEN 1 ELSE 0 END
      + CASE WHEN c5 IS NULL THEN 1 ELSE 0 END &lt; 2;