Do you already have the shrimp, or are you asking for shrimp you want to get in the future? If you don't already have them, I would recommend red cherry shrimp. Just make sure your tank is cycled and stable, with a few live plants and no fish that will eat them, and they'll just reproduce like crazy on their own. Moss isn't necessary, but they do like it. Whatever water temp you have for your fish should be fine as long as it's not too high.
For a 20Liter tank you could probably start out with 5 or 6, but they will breed like crazy and you'll have 50 before you know it.
