When I was involved in train planning (1977-83), the maximum number of vehicles permitted in a passenger train - without special authority - was 17....as laid down in the General Appendix.
The shortest sleeper working, IIRC, was the Kings Cross - Fort William portion departing Edinburgh Waverley at 04 35, consisting of three cars....SLS, SLF and BCK - although at one stage I believe it also conveyed a newspaper van of some description. However an extra two sleeping cars were added to the formation on Friday nights/Saturday mornings Northbound and Saturday nights/Sunday mornings Southbound. When the service transferred to the WCML to/from Euston in the mid-1970s, the Fort William portion was increased to four coaches by the addition of an extra seating vehicle. Perhaps
@Cheshire Scot could confirm or deny the above, as my memory is becoming a little hazy!