The three centre vehicles of a five car XC Class 221 have 188 of the 256 seats aboard, according to the XC seating plan on the internet.
Including the six stored sets, there are 38 5-car and 4 4-car sets available to XC. That's 84 end vehicles and 122 middle vehicles available. Unfortunately the only factors of 122 are 61 and 2, so you cannot make a uniform fleet by this method. However if you stack them into 30 sets of 4 and 1 of 2, you could make a nearly uniform fleet of 30 6-car and 1 4-car train for 31 units.
Given that the XC timetable currently only has 36 units available to run it, it could probably cover the diagrams.... or at least nearly.
The fleet of 62 end vehicles and 122 middle vehicles would represent a seat loss of 748 total seats, or around 7% of the 10,736 total available. This makes the assumption that all sets reformed would have one of each of the two kinds of end vehicle in use.
31 sets might not be ideal, because with the exception of coupling the 4-car to a 6-car, you would probably lose the ability to double them. I'd have to know how many diagrams (not including reinforcing units) there were to come up with a better solution. But losing ~7% of the seats for removing just over 10% of the vehicles ain't bad considering it would eliminate the need to couple them regularly and the associated staffing issues of having units in multiple. It would still result in significantly more seats than without the 6 additional trainsets.
I believe that (including the 220's) there's 44 diagrams which need to be run.
I believe that you need to have a minimum of 3 coaches in a set, so it "could" be possible to split that 4 coach unit to retain a couple of end units to create 2 sets with 3 coaches so XC could strengthen a few extra diagrams as 9 coach sets.
Although, arguably having the 221's as a near universal 6 coach fleet and all 220's (plus 1 set of a 221) as 4 coaches isn't a bad option.
30 sets with 6 coaches would likely be 25 (maybe 26) units in traffic (assuming 85% fleet utilisation).
That leaves 19 diagrams to be run by the 30 sets of 220's in traffic at any time, which would be enough to run 50% of the time as 8 coach sets, that's potentially just enough to cover much of the core.
The one thing which is worth bearing in mind is just how few seats the end coaches carry, yet have full lease costs, as such a small reduction in capacity might actually make XC more financial viable and so mean a replacement fleet is more likely.
Especially, if XC can show that, yes there's scope for a tiny uplift in capacity but at a significant cost vs a significant uplift in capacity for a slightly smaller cost uplift by getting a new fleet.
