First we have to confirm the list of digital high speed signals(Diff pairs),and rest of digital single ended(Controlled imp., traces) sginals.Differential pairs should be routed with specified trace width and air gap.Minimum of 100 mila pair-pair spacing to avoid cross talk
if it is any analog signals then this should be seperated from digital high speed signals (clock0to avoid the coupling ofswitching noise .
Also first confirm the board size....Also every signal layer(routing)need a adjacent ground reference place.Also if u are handling different power with higher current rating then we need a seperate power plane
No of layers should be EVEN
If any high speed interfaces(USB 3.0,HDMI,GIGABIT ETHERNET,SATA)then we nneed a separate Signal layers with Rogers or NELCO as Dielectric.(to minimize the dielectric losses)
preferably this can be on top(microstrip line)to have high speed propagation speed.
While designing stack up copper weight should be balanced to avoid the warpagee
ie -------------------top(routing)
-------------------(ground)inner 1
------------------------------------------------------------------------------------------
--------------------power-inner2
--------------------bottom(routing)
if split the stack up into 2 equal parts then the layers should be mirror image of another part
Based on this select the no of layers
regards
Rajan .K