Rule of thumb is 1 inch fish when fully grown per 1 gallon. 19 litres roughly 5 gallons, so 5 inches of fish. Guppy fully grown is 2 inches. So 2 to 3 fish.
You also need to accommodate fish in suitable numbers. So minnows are social fish and need to be in groups of 6 minimum. 10 would be better. Minnow 1.5 inches. 6 minnows, 9 inches. So say a 10 gallon (40 litre) tank which will also give them enough swimming space.
Its a rule of thumb, doesn't work when talking big fish. You couldn't put a 10 inch goldfish in a 10 gallon tank. 1 inch per gallon is conservative with small fish too, but for a beginner its best not to push it.