I'd say start with six and see how things pan out with filtration. A tiny tank like that can become a toxic "puddle" in the blink of an eye.
Given how quickly toxins will build up, fast growing plants would be a great asset, to soak up ammonia and nitrates (but not nitrites, these need bacteria to process). I'm not green fingered enough to suggest anything suitable for a 21l, but perhaps a thread in the planted section is worth a punt?