You surely realize that no human being can be available 24/7 within 20 minutes. It's beyond slavery to expect that from any employee.
You need at least 10 sysop/sysadmin to achieve anything close to that SLA, with a sustainable rota. Contrary to the parent posters who believe it can be done with THE right guy.
With 3 people you can have a "follow the sun" rotation during business hours which takes care of the entire week, and I don't think you would need 7 more people for the weekend.
