SQL Server requiere subconsultas que usted SELECT FROM
o JOIN
tener un alias.
Agregue un alias a su subconsulta (en este caso, x
):
select COUNT(*) from
(
select m.Company_id
from Monitor as m
inner join Monitor_Request as mr on mr.Company_ID=m.Company_id
group by m.Company_id
having COUNT(m.Monitor_id)>=5) x