When I took the Sleeper from Helensburgh to Westerton last week, I estimate there were about 6 people already in the seated section. We easily got a bay of 4 to ourselves. I think everyone got off at Westerton. We got a whole coach to ourselves from Westerton to Glasgow.
I've taken this train several times and the only time I've known it be full & standing was on the penultimate weekend of Class 37 haulage on a Bank Holiday weekend. There were people sat/stood all over the guards van, and of course the front vestibule was well loaded too. But that was very foreseeable. I remember some American tourist being really annoyed there was only one seated coach, and we let a load of tourists who were sat in our booked seats remain seated until around Ardlui/Helensburgh, but they were most unhappy to stand from there to Westerton/Edinburgh.
Chances of it being full are highly unlikely.