You can use a tally function, number table, or similar to get a selected # of rows to generate -- e.g.,
DECLARE @fquantity INT = 50
SELECT NULL AS Col1, NULL AS Col2, NULL AS Col3, NULL AS Col4, NULL AS Col5
FROM [util].[Tally] (1,@fquantity)
The union all would come into play if you need to combine returning existing data w/ blank rows. If you need the total of existing rows plus blank rows to equal fquantity (I'm not clear if that's the case or you're only trying to return blank rows), you would need to subtract the # of existing rows from fquantity, and union all the existing rows w/ the generated rows.