Yes, one chair per floor should be ok in principle, I presume you have lifts that can give access for wheelchair users to the upper floors and the evac chairs are part of the strategy to get most of them out safely.
Think about the number of refuges and their location, refuges dont have to be in the stairs, a floor divided by a fire resisting wall each half with a staircase can be counted as having two refuges. Think about where the chair is sited- is it always accessible-if only one per floor can the fire warden for the floor take it to the location where it is needed, and who will be trained to use the chair and who will take the users own wheelchair downstairs for transfer back at the bottom of the stairs.
Remember that evac chairs are only one piece in the "special needs" jigsaw. And if theres no way for wheelchair users to access an upper floor then theres no need for evac chairs but still a need for refuges for people with other types of disablement.
