How much water do betta fish need?
You shouldn't keep a betta in anything under about 20 litres. The shops will often tell you that 5 litres is enough but it's really not.
You'll want to have at least 20 litres for a few reasons. Firstly, as the fish is always creating waste, the more water you have per fish, the more diluted the waste is in that water. More water, less toxic ammonia (or nitrates) hanging around in the water.
Secondly, small volumes of water are notoriously difficult to keep at consistent levels (of pH, KH, temperature, etc.) Even 20 litres is still a very small amount of water. Anecdotally, I had a a 20 litre tank initially and it was far harder to keep at consistent levels than my 180 litre tank. Get the biggest one you can realistically stretch to.
Remember, getting a fish is like getting any other pet except you also have to provide the environment for it too. If this is your first fish or you don't know what the nitrogen cycle is then please do your research. This hobby can be incredibly rewarding but it can also be incredibly upsetting if you get it wrong.