If you have Acrobat 6, then you can add page numbers very easily (Document, Add Headers and Footers...). (I'm afraid I don't know whether this is only available in Professional or whether it's also available in Standard - I have Professional, and it is available there.)
Otherwise, if you're using Acrobat 5, you could do it with JavaScript and fields. Not sure exactly how, though. It'd probably take me a few hours to find it or work it out and I haven't ever needed to do it. Check out the JavaScript guide that comes with Acrobat 5 where, with a bit of work, you will probably be able to find the solution.