I was under the (foolish) impression that 6 layers is the limit
We have a 10 layer board made here.

Wonder what kind of circuit complexity will call for 20 layers - or is it one of those "if it can be done , it will be done " stuff ?
Not really, it is for a space constricted unit that is not, but could be described as, a PC with 12 serial ports, GPS, ethernet, display, USB, Smartcard and quite a few more things.
When you get a lot of stuff in a small place, there is not much room for wires and many more crossings occur.
Another reason for many layers is that, if you use less layers, you will very fast fill your board vith vias and 'funny' routings, and at high clock speeds that will give you trouble.

Erik