It will never end, and there's no real right or wrong - even the industry definitions of 'how many doors' is confusing.
My own take on it is:
Fragrance & Cosmetics House: Established primarily to make fragrances and cosmetics. Many of these houses have 'mainstream' distribution as well as more limited Exclusive Collections.
Guerlain, Caron, Coty, Lauder, Shiseido.
Designer: Companies that make clothes and/or jewellery etc. and also have perfume as an ancillary line with mainstream distribution. In the last decade or so many of these designer houses also have launched Exclusive Collections with limited (often only 'in-house') distribution (Hermessences, Chanel Exclusifs, Dior Prive, Cartier Les Heures etc.)
Dior, Gucci, Armani, Hermes, Tiffany, Chanel, Comme de Garcons, Tom Ford
Niche: Smaller companies, most founded from the late 60's onwards, and in many cases starting with one boutique helmed by a perfumer. Many have gone on to become quite large businesses, arguably making the transition from Niche to Perfume & Cosmetics House.
First Generation: Diptyque, Maitre Parfumeurs et Gantiers, L'Artisan, Parfum de Nicolai, Annick Goutal.
Second Generation (These ones had a business plan): Frederic Malle, The Different Company, By Kilian, Divine, Histoires de Parfums, Maison Francis Kurkdjian
There are exceptions to every single one of the above, of course, and other anomalies, but this is how I keep tabs. It's quite fluid really - interesting to watch unfold.
A confession - when I see people call Guerlain 'designer' MEGO