I have lots in two of my tanks, a 32L/8.5G and a 120cm/4' 180L/50G, both with gravel substrate.
About a year ago, put 1 adult snail in the 32L, 4 adults in the 180L. There's now an uncountable number in both tanks, though none have grown to more than about half the size of the original adults, or developed their black shell. At least 2 adults have died in the 180L, I pulled their empty shells out recently.
A friend of mine put 1 or 2 trumpet snails in his ~40L tank, but they haven't reproduced, must be male.
I don't overfeed my 180L tank, none of the food I put in there makes it to the substrate, the snails aren't eating plants, they don't appear to be eating much if any algae, so I think they're mainly surviving eating fish poop. That's okay with me. I see very few of them during the day, have to look pretty close to spot any (I just walked over, took me about 30 seconds to spot 2, one of them a dead empty shell), but after dark they emerge in their hundreds and crawl all over the substrate, plants, rocks, driftwood and up the glass.
All things considered if I was starting the tank over again I'd probably put them in.
The 32L tank is a slightly different story, much of what I said above goes for it as well, but I don't think they're as beneficial in a small tank with small fish. And lately, possibly because there's only a gold panchax killifish, single rummynose tetra, and a 7cm native macrobrachium shrimp, they've been coming out while the lights are on and are about as much of an eyesore as a tank infested with pond snails, if you don't like that sort of thing.
If I was setting up this tank as is again I wouldn't put trumpet snails in it. But, I'm planning on changing the substrate in this 32L tank to sand (possibly today...), and for that reason I'll keep about half the snails in it, give the other half to a friend of mine who recently set up a 6' 315L/83G tank with sand. He's said the sand really shows up the fish poop sitting on top of it, we'll see if the snails do something about that for him.