как-то так ...
WITH [12,-9,30,-5,4] AS coll
RETURN filter(x IN coll WHERE x > 0) AS pos
и если вам нужно фактическое количество положительных чисел
WITH [12,-9,30,-5,4] AS coll
RETURN size(filter(x IN coll WHERE x > 0)) AS pos
и как @christophewillemsen говорит
WITH [12,-9,30,-5,4] AS coll
RETURN size([x IN coll WHERE x > 0]) AS pos