Keynote advanced slide numbering "X of Y"

X is possible. From the Inspector Window, look in the slide tab (second from left) and click on appearance on the top. There should be an option there for "slide number." This option may need to be checked for every slide, which you can do all at once by highlighting every slide in the left frame (click on one slide, then command+A) and then going through the inspector to option in the "slide number."

As for Y, you may have to place that in manually using a text box.

And to follow-up on Edward's post, it is best to do this all on the Masterslides so you don't have to do it with each slide, just one of each master that you use.