It is 16:
8 from Inverness, consisting of a mark 2 seated brake carriage (2+1 seating but officially Standard Class), a mark 2 lounge and 6 Mark 3 SLE/SLEP/SLED
5 or 6 from Aberdeen- again with the mark 2 seated and lounge cars and 3-4 SLE/SLEP/SLED
3 or 2 from Fort William, just the sleepers (additional day carriages go only between Fort William and Edinburgh).
The Fort William sleeper cars are marshalled with the Aberdeen portion to create a formation that consists of two 8 carriage sections.
The Edinburgh and Glasgow lowland sleeper sections have the same formation as the Inverness portion, forming a 16 carriage train south of Carstairs.