I think 2 litres will be fine.
It needs to be mixed 50/50 with water. Tap water is ok, distilled or deionised water is better.
Mix it before putting it into the car. That will guarantee the mixture proportions. Putting water and coolant in separately and expecting them to mix is a bit random. It will take time to mix thoroughly, and may give the wrong concentration.
Less coolant reduced its effectiveness at corrosion inhibition, and reduces its anti-freeze properties. Too much actually reduces its effectiveness too.
Get a pot of some sort. Big plastic measuring jug, (Cheap but often don't pour well, so get a funnel too) or similar, or an empty 5l pot, (clean inside) and mix the water and coolant 50/50. Then pour into the car.