How would the number of SAs have anything to do with the number of users? I don't get that one.
Unix boxes (superdomes and Suncrap) I use 1 senior per 20 or so with another junior for gruntwork. Also depends on the complexity of the Pars and storage. I try to group the guys in relation to the type of applications living on the boxes, as they are apt to be more familiar with the tuning requirements. I also require them to work closely with the apps teams in a sort of architectural role during the design phases of home-grown apps. We catch a lot more crappy java that way.
I have asked the same question for years. From Gartner, Giga, whoever. There's no magic number.
Budget adjustment time already?
edit: Oops, forgot AIX. About 1/10. I'm out of the Wintel world anymore, but it always seemed to require more babysitting. Although the MCSESAMOUSEs claimed differently.