Keeping 100's of them would be difficult anywhere in the world, since these are fish that need individual tanks if they are to be kept in show condition.
I have 26 of them and they take up time due to tank cleaning and water changes.
However what I noticed when I went through posts on this forum is that many people have one or two bettas that live in large tanks with a lot of support equipment. In Thailand hobbyists tend to have collections of Bettas and keep them in small tanks with no filters, heaters, air pumps or tank decor.
The tanks are seperated with cards which are removed temporarily to show off the fish in their splendid fighting stance.
I'm not saying I agree with bare mini-tanks since I am more sentimental towards animals as a Westerner. But still the difference in Betta care here, where these fish originate from, is remarkable.