Class 397 could be lengthened in the future. There’s also talk of some other operators placing an order. My opinion? It’s such a good unit that there won’t be 12 of them for long!
I wouldn't be surprised if they have an extra
coach or two built for them, or maybe an extra
couple of units built to beef up the Liverpool service, but where else (beyond TPE's WCML service) is there going to be demand for a pure-electric five coach 125mph train?
Most services that are important enough to require 125mph running will need more than five coaches.
XC and EMR can't take pure-electrics (and EMR are ordering something from Hitachi)
LNER and GWR will want something compatible with what they have (Hitachi), and will probably want something bi-mode, given the unelectrified extremes of their networks - why order a tiny non-standard fleet that doesn't offer anything that the 800/801/802s already provide?
HT are ordering Hitachi (what with Hull not having any electricity)
Other TOCs who have EMUs won't find it worth paying the premium for 125mph stock (the space taken up by "crumple zones", the lack of corridor connections)
There might be twelve more 397 *coaches* ordered but I think they'll stay a pretty tiny fleet (which makes long term future a bit uncertain - given that HS2 may well see a lot of 390s freed up to run the routes that the 397s do).