My approach using the comments above:
SELECT DISTINCT COUNT (*) OVER (PARTITION BY B.ZPTIDHUIS, B.ZPTIDAFD) AS CltAfd,
With myCTE AS
(
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