Counting no of rows returned by a select query

SQL Server requires subqueries that you SELECT FROM or JOIN to have an alias.

Add an alias to your subquery (in this case 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