@kaputnik The 158's used for Highland services (which migrate to the south for various diagrams) have no cupboard (only 4 units might remain so equipped, - gained by Scotrail from England about 3 years ago) Highland ones have a seat of seats removed in the 'other' coach. My record for a Northern Rail train with 140 seats filled by 78 passengers was having 8 bikes in the bike space (ie 10% of passengers with bikes) I would suggest that this is a commonly found percentage on routes popular with cyclists. After PfS I observed an arrival at Glasgow Queen Street when 32 bikes came off a 6 coach Class 170 (that would be 8 bikes in bike spaces and 2 bikes per vestibule in each of the 12 doorways on the opposite side to the platforms.
Basically for a 130-seat 'Highland' Class 158 6 bikes can comfortably be fitted on the unit and approximate to 5% of the seated capacity - or 10% of a half filled train - typical for off-peak travel on most trains I've checked.
Records for Scotland as far as I'm aware - Class 334 (130 passengers 13 bikes 260 seat trains - Ardrossan harbour 14.15 service - common) Class 314 37 bikes on 3 coaches for Loch Lomond path opening, 36 bikes on Class 380 etc. We do need to press for flexibility in internal layouts that permits bikes to go on board and internal layouts that (despite Scotrail's position on this) that allow tandems and other special types of cycle to fit on board. If you do get them on take pictures! and do count the bikes on the trains!