You only need one or two in a box, usually one. The main use of a dummy is to use up the last bit of space at the end of the box; when fitting or removing it will be the last or first so that the other frames are easier to loosen and remove or replace, with that little extra space made by initial removal of the dummy.
If I want to reduce the size of a box I use a close-fitting divider. They retain heat in the reduced brood area better than dummies. Harder to remove, but better suited for purpose. I rarely use a divider for that purpose - putting them in a smaller box is usually a better option, but dividers at both ends (one has an entrance clearance at the bottom) of the over-wintering area of the Dartingtons is a design point - so the cluster is always well away from the hive ends.
Regards, RAB