Systems administrators under UNIX are required to perform a wide variety of tasks. In some of the larger installations, there may be tens or even hundreds of people involved in the administrative and support functions, but for the vast majority of UNIX systems, the systems administrator is just one of the users who has used UNIX more than the other users, or a programmer who became the administrator by an accident of fate.
As a rule of thumb, about 5-10% of the computing budget of most companies is spent on systems administration and security. That means one systems administrator for every 10 users, or almost one hour per day per user spent in systems administration. In large installations, systems administrators only perform a small number of tasks, while in smaller systems, the administrator may take on many roles.