Ive done the same went from atx, to large full tower, to mid tower.Gone through 5 in 11 years! Started large! And gotten smaller!
6 if you include the pre-built ones from 20+ years ago (huge Gateway case, slim Compaq case), otherwise 4:
Antec P180 (2005, built like a tank, still in use for a family members PC)
Fractal Define R5 (2015, still used for my main build)
Be quiet! Pure Base 500 (it needs to take advise from the brand name, has an annoying rattle, used in a space PC)
Fractal Define S (got this new for £20, it's big but not as well built as the Define R5, used for a server PC)
Add a Lian-Li Lancool 216 to the list.First was a Dell prebuit.
Then i had cases from the following brands.
Fractal
Coolermaster
Lian-Li
2 X Phanteks (including the Pro M i have now)
So that is 6 in around 25 years, but am toying with the idea to get a new one soon.