I think it depends on how often you use them, what you use them for and your stamping style. I use mine a fair bit, so I have quite a few - between 25 & 30. I see a lot of embossing folders out there that I like, but having as many as I do, there aren't too many these days that I "need" KWIM? I do have a few on my wishlist - like getting a woodgrain one, but for the most part, I have something that will work. I have a tendency to buy more generic type patterns though, that can be used for anything. Like the d'vine swirl, a swiss dots, a herringbone, a flowery one with small flowers, a flowery one with big flowers, a heart, a star etc. But I have found that the larger my collection has grown, the fewer new releases I think I need.
I do think if you use them often, it is necessary to have a good variety. But I don't think you need them all.
"You may not have lost all your marbles, but there's definitely a hole in the bag." Grumpy Cat