If you want to see the count of all users under all admins use this
SELECT Admin, COUNT(username) as "Number of employees"
FROM Users
GROUP BY Admin;
if you wish to show list of users under single admin you need to brushup your SQL basics. the simplest query ever.
SELECT username
From Users
WHERE Admin = 'xyz'
Make sure that your are not copy the entirecode, but along with that provide correct colom and table name