Prova detta (modifierat för projekt utan erbjudanden):
SELECT
Project.addDate,
Project.idOwner,
Account.Username,
Project.idProject,
Project.Price,
ISNULL(q.offercount, 0) AS offercount
FROM
(
SELECT
o.idProject,
COUNT(o.idProject) as offercount
FROM Offer o
GROUP BY o.idProject
) AS q
RIGHT JOIN Project ON Project.idProject = q.idProject
INNER JOIN Account ON Account.idAccount = Project.idOwner
ORDER BY addDate DESC