My approach using the comments above:


    With myCTE AS
     (
        SELECT    DISTINCT COUNT (*) OVER (PARTITION BY B.ZPTIDHUIS, B.ZPTIDAFD) AS CltAfd,
                  B.ZPTIDHuis AS IDHUIS, B.ZPTIDAfd AS IDAFD
        FROM      Bewoners_STAGING B
        WHERE     B.ZPTCalc <> 'n' AND B.AfdMeetellen <> 'nee'
     )
    
     UPDATE    B
     SET       B.AantalCltn = C.CltAfd
     FROM      Bewoners_STAGING B JOIN myCTE C
                    ON B.ZPTIDHuis = C.IDHuis AND B.ZPTIDAfd = C.IDAfd