I have an emp table with the records below:
emp
INSERT into emp(EmpId,Emp name, Manager) Values(1,A,M1) values(2,B,M1) values(3,C,M2) values(4,D,M3)
select manager, count(*) as employees from emp group by manager order by count(*) desc
Take the first record. Depending on your SQL version, you can do this with a limit statement.