The total number isn't relevant. Most admin work is done by a small number of very very active admins maybe about 10% of the total admins at any given time.
My first question stands: why are they overworked?
I'm not sure they are, but if they are then it's because not enough admins are helping out. That could just be that people get bored and move on (I think the rule of thumb for only communities is that people leave after about 18 months - considering we require editors to have been around for at least a few months before they become admins, that doesn't give them long) - that could be extenuated by them discovering that a mop isn't actually as interesting as they thought. The alternative is that admins are mistreated so much that they can't really be bothered any more.