I am also in agreement that you cannot put numbers to it. I would say that you need enough to get the job done without burning out the DBAs that work on it. A point that some managers fail to comprehend is that DBAs are human too. A well rested and happy DBA is worth -IMHO- at least 2 or 3 burnt out versions.

Just so readers do not get the wrong opinion - I am extremely happy with my current situation, but, I have been on the other side of the fence in my career.

Joe