Many years ago I lived on a large housing estate in Scotland where there were hundreds of residences like that. Terraces, for the most part. Each unit in the terrace had a common stairwell with two residences downstairs and two residences upstairs. The residences were numbered sequentially, as houses would be. They weren’t flats or units but pseudo-houses. You’d have an address like 11 Furby Grove, not Flat 11 or Unit 11 Furby Grove.
There are a few units (far smaller) a couple of streets away from where I am now, in Wales.
I suspect the numbering scheme used depends on whether the building was divided as-built or after-the-fact. After-the-fact and all you can do is call them flats, or go with 11A, 11B, or 11.1, 11.2, 11.3. But if you’re designing it that way from the start you might as well number the residences sequentially.