We have two small goldfish in a tank, our filter broke a little while ago but they seem just fine. They need to be cleaned out regularly. In the olden days you would just have a bowl and no filter wouldn't you.
Pets at home are pretty fascist though. Our local aquatics shop was happy to recommend the fish we have, but Pets at Home who sold us the tank said they would only sell us some teeny tiny thing, only after we had had the tank for a week.. They had dead fish in their tanks too
No. You absolutely need a filter. it's not just to save you from doing so many water changes, it houses bacteria that process fish's waste into less harmful nitrate. otherwise they are swimming around in ammonia soup which will lead to their demise, sooner or later.
The absolute bare minimum you can get away with is a filter and a tank- anything smaller than 45 litres is too small for fish. You can buy and run a set up like that for next to nothing if you look around enough.