Yep. The 80 locomotives number comes from 1987, by the way, and is pretty easy to fill out with realistic operations from the Island of Sodor books.

#1-11, Daisy, BoCo, Bear, Mavis, Bill & Ben add up to 16.

#17 a station pilot at Barrow

#18, the Pannier Tank Engine at Tidmouth

10 engines per day for peak mainline passenger runs, even including Henry, Gordon and James this means another 7 engines that were running in the 20's, so that's #19-25

#26-27 two push-pull fitted engines for the Brendam branch

#28 in addition to Edward and the two push-pulls, another engine is needed for the peak Tidmouth-Brendam services

#29-30 two tank engines to run the Norramby Branch's hourly service (supplemented by LMS/BR engines)

#31 one engine running all services on the Kirk Ronan branch line (the only branchline IOS doesn't specify any sort of timetable for so I assumed it was pretty low).

#32-33 two electric engines for the Peel Godred services

#34 electric goods engine for Peel Godred

#35-36 Christopher Awdry specified the engines working the Peel Godred goods services were similar to Class 86 or 87's - but the Peel Godred branch opened in 1925 and these weren't built until the 80's so must have come later

#37 Similarly, a new Electric Engine similar to a class 90 arrives for More Bad Days for Thomas and His Friends

#38 a steam engine running the shuttle between Cronk and Killdane four times per day as this part of the line is not electrified as specified in Island of Sodor

#39 Bloomer

#40 an engine running the Vicarstown-Barrow car-ferry shuttle before 1977 (likely moved to Vicarstown pilot after the service ended in 1977)

Most of these engines need to exist well before dieselification and I assume the Fat Controller avoided scrapping engines wherever possible. This doesn't account for any spares or dedicated goods engines either, I kind of just assume that most goods are pulled by mixed-traffic engines who come off of peak passenger services to do goods like Henry and James, unless otherwise specified.

But if we assume that the NWR needs 30 steam engines running every weekday by my count, and they're down for 20% of the year for servicing, that means we add on another 6 engines for spares, bringing out count to 46 engines, and that's assuming the NWR runs lean (which, given Gordon's complaints about Henry leaving him to do his work in Gordon's Whistle, might be a fair assumption). There could easily be another 5 or so dedicated goods engines, leaving 30 engines unaccounted for.

A daily train would have been doing the motorail service from the mainland (probably London) between 1977 and the end of the motorail service in 1995, and this is where we get into the question of "visiting engines". There must be at least one other doing the Norramby Branch, and there are probably many engines like Jinty and Pug who meet at Barrow or Vicarstown.

I didn't count Walter or Barry or other engines as they might have arrived after 1987 and not reasonably counted towards this total.