Du kan göra aggregeringen i en CTE och sedan gå med på det
WITH Children(Cnt, ParentId)
AS (SELECT COUNT(*),
ParentId
FROM dbo.Post
GROUP BY ParentId)
SELECT P.PostId,
P.[Body],
ISNULL(Cnt, 0) AS Cnt
FROM dbo.Post P
LEFT JOIN Children /*To include childless posts*/
ON Children.ParentId = P.PostId
ORDER BY P.PostId