I think this depends greatly on the infrastructure you have in place. If you don't have a good infrstructure the number of servers that any single DBA can handle is greatly diminished. Just think about patching, how many servers can you patch at once manually? I can think of all kinds of monitoring that you might need.. But just off the of my head..

1. Services running, SQL and Agent.

2. Services active, SQL responds to logins, Agent is running jobs.

3. Errorlog is monitored for issues.

Any other thoughts?

CEWII