zepertino
I'll try to share with you some of my expirience.
First of all you need to determine the number of power layers, it depends on the voltages you connect to the IC's. For example if you use only 3.3 Volt devices and all of them use digital power (no analog IC's like power convertors or analog opamps), then 1 power layer will be enough. If you have 3.3 Volt and 5 Volt devices, and also 3.3 Volt analog devices you might need 3 power layers, but if you have 1 or 2 5 Volt devices in this case you can connect their power using thick conductors - then you don't need 3 power layers, but only two : one digital 3.3 Volt and one analog 3.3 Volt. This way you decide how much power layers you have.
Now the Ground layers. If you have only digital devices you need only one GND layer.
If you have also analog, you might want to split the Analog GND to a separate GND layer. Now let's say you have 1 digital power 3.3V, 1 analog power 3.3AV , digital ground GND and analog ground AGND. It is already 4 layers.
Now we go to the layout itself.
If you have BGA devices on the board, check how much conductors you can pass between the BGA balls. Some of the you can pass 2 conductors, some of them 1. Usually you can find this information in the device datasheet or application notes. You can also find there how many layers you need for layout of the BGA device.
Any other device has less limitation than BGA and you can manage with 2 layers for it's layout. For our example let's say you habe fBGA device (fine pitch BGA) so you can pass only 1 conductor between the balls/vias of this BGA, and you need 4 layers for it. So now you have 8 layers : 4 Power and Ground and 4 for signals.
Hope it helps.