This generates a Tally table on the fly with zero reads.

That's right. Well, actually it generates 145 because of the SELECT 0 UNION ALL.

That's right, it's known as the old-join syntax which creates a cross join because there's no condition to join.

I'm using (SELECT NULL) because ORDER BY in the OVER() clause doesn't admin constants. I could use anything but the important reason is that I don't care about the order, I just want to get consecutive numbers from 1 to 144 ()in this case. You can check what it does if you run that part of the code. Here's how I test cascading CTEs:

WITH E(N) AS(

SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL

SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL

SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL

SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 --12 rows

)

--,cteTally(N) AS(

SELECT 0

UNION ALL

SELECT ROW_NUMBER() OVER(ORDER BY (SELECT NULL))

FROM E, E x --Enough for 12 years

--)

--SELECT ClientID,

-- MONTH( DATEADD(MONTH, N, ISNULL(StartDate,'20140101'))) MonthNum,

-- YEAR( DATEADD(MONTH, N, ISNULL(StartDate,'20140101'))) YearNum

--FROM cteTally t

--JOIN dbo.Client_Clients c ON DATEADD( MONTH, t.N * -1, GETDATE()) >= StartDate

-- OR (DATEADD( MONTH, t.N * -1, GETDATE()) >= '20140101'

-- AND StartDate IS NULL)

--WHERE ExitDate IS NULL

--AND N % SheetsRequiredNumMonths = 0

----AND ClientID = @ClientID

I don't believe that those are stupid questions since you had most of the answer and it's good to be sure about the code you're implementing.