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