I made some corrections... I tried to create single query.

 

SELECT 

(DATEDIFF(d, @startdate, @enddate)/7)

 +  CASE 

WHEN DATEPART(dw, @startdate) = (6)

OR DATEPART(dw, @enddate) = (6)...

This one should work. It is single query.

You can change number (6) to number of day that you need to count. Friday = 6 when SET DATEFIRST 7.