ScotRail won't be storing anything. I'm sure there will be a couple of spares kept at Haymarket(?) but only just enough to cover failures. They will lease whatever they need from the ROSCO. If they want to increase to 2+6 they will simply lease more coaches. It's up to whoever owns them to store them.
Remembering of course they will be refurbished before going north of the border, including getting power doors, so there would be a bit of a lead time to leasing more.
Cheers
David