The idea came up in this Hacker News thread, commenting on a 37signals interview, that having three system administrators is the minimum optimal number of admins. Everyone wants to lower their costs by having each admin administer a lot of machines. The problem is when you have fewer than three admins you can never get a break from the constant corrosive pressure of always being on call. When every moment of your life you are dreading the next emergency, it eats at you. Having three admins solves that problem. With three admins you can:
- Go on a real vacation. The two remaining admins can switch off being on call.
- Not be on call all the time.
A larger shop will naturally have more admins so it's not as big an issue, but at smaller shops trying to minimize head count, carrying three admins (or people in those roles) might be something to consider.