We find 4 and 8 layer boards are the best for most of our RF designs (depending on frequency, complexity, etc.) - more layers than that and your dielectric layers start to get too thin and then your tracks have to be super narrow to hit a 50 ohm impedance... But of course, that also depends on what type of substrate you're using, etc.

But for complex digital boards, 16 (or more) layers is common.