From what I understand of Scotrail's cascade plan, the additional 8 334s will be moved gradually onto the new route as the cascade unfolds. I think it allows some degree of flexibility as the 320s go off to get refurbished and the 380s start to enter service.
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OK, I've worked out some more with some information found on Scot-rail.co.uk.
There are to be 36 Class 334 diagrams in service from December (including 2 operational spares), all on the North Clyde Line & through to Edinburgh via Bathgate. If rail-britain's number of 28 is for Airdrie-Bathgate only, that leaves 6 units for Springburn and Airdrie terminators. The remainder (at least 7 diagrams by my reckoning) will be operated by 320s.
There will be 15 Class 318 units on the Argyle line, rising to 18 in April once the last of the 380s enter service. I don't know how many Argyle Line diagrams there needs to be. Based on the current timetable, I would estimate 24 diagrams (again, diagrams thanks to Scot-rail.co.uk). So you'd need at least another 9 units, these being Class 320s, with perhaps some more longer trains as well (which seem to be needed on the Argyle Line). Of course, over the next few months, you're going to see longer trains on this line to combat overcrowding.
There are 20 Class 320 diagrams, split between North Clyde and Argyle duties. So, if at least 9 are for Argyle, at least 7 are for North Clyde, that leaves four units as operational spares or for strengthening.
Many of these calculations are made by myself, and consequently may be catastrophically wrong! (EDIT: They are pretty catastrophically wrong!)