WITH
   alias AS (...)
IF @TopCount>0
   SELECT TOP (@TopCount) * FROM alias;
ELSE
   SELECT * FROM alias;
GO

CREATE PROC dbo.if_branch (@top INT = 2147483647)
AS
BEGIN

WITH d AS
    (
        SELECT *
        FROM sys.databases AS ds
        WHERE ds.name = 'tempdb'
    )
SELECT TOP (@top) *
FROM d;

END

IF @TopCount IS NULL OR @TopCount < 1
    SET @TopCount = POWER(2, 30) -- A big enough number (careful with overflows!)

WITH
   alias AS (...)
SELECT TOP (@TopCount) * FROM alias;

WITH alias
  AS (...)
SELECT TOP (CASE WHEN @TopCount>0 THEN @TopCount ELSE 2147483647 END) *
  FROM alias;

declare @TopCount int

SELECT
    @TopCount = CASE
        WHEN ISNULL(@TopCount,0) > 0
            THEN @TopCount
            ELSE count(1) END
    FROM alias

select top (@TopCount) *
from alias