Anything with RF sensitivity can benefit from isolating ground layers. You can use layers for managing thermal effects. Vertical isolation of signal groups to reduce cross-talk. Horizontal space restrictions.

And then of course, signal count and density. Many FPGAs have a thousand or more pins.

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.

Your comment reminds me why it always pays to consult with an expert, pennies up front versus real dollars later.

Modern BGAs will often need that many layers just to break out all the signals.

Once you use a BGA with X balls in Y rows and Z columns with so and so pitch the minimum number of layers needed quickly goes way up.

If you use any kind of BGA you will want to start with 4 layers just for signal integrity.

Google something like "bga escape routing" -- I found one chip that needed 12 layers pretty quickly.

You might be able to route it but that doesn't mean it will work due to crosstalk and then those additional layers will come in handy as ground.