Ah, this takes me back to my undergraduate days in Exeter! In the '85 - '86 timetable (and others) there were trains at 09.40 and 13.40 from Paddington to Penzance arr 15.40 and 20.01, returning at 09.32 and 16,17, arriving 15.37 and 22.09. They had a very good restaurant service, so the 09.40 did breakfast and lunch, and its return at 16.17 afternoon tea and dinner - one catering crew all the way there and back, who worked one day on, one day off.
As 455D says, could be up to 14 coaches but the relatively interesting thing was that there was in effect a portion of about 5 coaches that was sometimes detached/attached at Plymouth. So, on summer weekdays, the 09.40 was long to Plymouth and short to Penzance; its return at 16.17 was short all the way. The 13.40 was usually short all the way down while its return, the 09.32 the following day, was long. When the 09.32 from Penzance arrived at Paddington the portion was detached and the main part of the train formed the 16.25 to Cheltenham and return the following morning at 08.30, after which it could go back down west on the 13.40. Since neither of the up trains arrived in London until both the down ones had gone, there must have been a third set of "main" coaches as well: I never noticed these on any other workings so I suspect these gave the breathing space for maintenance. The 13.40 called at Keyham, Saltash, St Germans and Menheniot, so was a little long for the platforms there...
More interesting was that the portions also formed the seating portion of the Paddington - Penzance & return sleeper. The portion that was dropped off the 0940 at Plymouth later worked forward as the 16.35 all stations Plymouth - Penzance and then came up on the sleeper. The portion that had come up from Penzance on the 09.32 in the morning, once separated from the main bit that was off to Cheltenham, went down to Penzance on the sleeper. Obviously that evened out as it was just a swap of the two portions between London and Penzance. On a Friday the 09.40 ran through to Penzance as a long train, i.e. kept the portion beyond Plymouth; the 16.35 Plymouth - Penzance was covered by the FO Brighton - Penzance. On summer Saturdays the trains were long throughout in both directions. I presume that there was also a third "portion" that allowed time for maintenance - these were worked fairly hard as they were on both the day and the night trains, the latter still running every night of the week at the time.
Based on the timetable, rather than my knowledge from the time, they also ran on Sundays: 11.45 and 18.45 from London; 13.10 and 18.40 from Penzance. There were tweaks at other times too, e.g. the 13.40 from London including a portion on a busy Friday, in which case there would have been some knock-on alterations in order to keep the coaches, particularly the portions, in the right places. (E.g., as a complete guess, for this example one of the down trains on the Sunday would go short and then come back long with the extra portion that had found its way to Penzance.)
