SELECT R.field1, R.field2, R.field3, R.field4, COUNT(B.item_ID) AS howmany
FROM rentalurunler AS R LEFT JOIN bookings AS B
ON R.itemID = B.item_id
GROUP BY R.field1, R.field2, R.field3, R.field4
You need to explicitly list the fields your are SELECTing, because (excepting for the COUNT field) they need to be listed again in the GROUP BY. And, in any case, it's bad practice to use
in most cases.
NOTE: You use a LEFT JOIN if you want to find counts of ZERO OR MORE. If you do not want to see counts of ZERO, then change the LEFT JOIN to INNER JOIN.