I used to handle about 400 hundred people and 400 boxes as a sneaker admin. So two folks for PCs and one for servers is workable -IF- you have your act together.

My suspicion is that you all are a bit low. Reasons:
- People get sick.
- People take vacations.
- You need to cross-train each other.
- Special projects can and do come up.
- There are under-met needs in the company.

To get the person that you need, you really need to show the business case for it. Once you can do that, ratios don't matter.

Start by going to a few marriage councilors. I'm serious. Good marriage counseling is cheaper than a divorce by an order of magnitude and infinitely more helpful. Quite simply, it's a bargain. We went to two difference councilors. One taught us skills and the other ran us through a battery of tests, and then reviewed our weaknesses. Both were informative.

Above all, your first job is to show how this is THEIR problem. You need to show the management how this issue is THEIR issue, will bite them in the ass, and that their best course of action will be to pony up the money. Politely, of course. You don't need to be an ass. The second thing that you need is an ally. That's your boss. You need his help. You need him working the money/politics side for you. If not him, then you need someone who knows the money politics side. Don't point fingers. Your job is to come in, straighten things out, and bring the shop up to a PROFESSIONAL level. Keep that as your attitude, and make sure that everyone knows it.