The plural of "fish" is "fish," not "fishes." When one uses the word "fishes," it does not convey a plurality of fish but a plurality of types of fish.
If the fish are small fish, then the rule of thumb is one fish per gallon, which a liter is about one quarter of, so definitely not one fish per liter. You do that, you're going to have an overcrowded tank, causing the fish to become aggressive and attack and eat one another. Also, it will be difficult to keep fish alive for very long because of amonia buildup. Pet stores have large quantities of fish in tanks, but the fish are only in there a short time; the shop is well-versed in maintaining a chemical equilibrium in the tanks, which tend to be large and so more resistant to chemical fluctuations; and even then you often see multiple dead fish in aquariums at pet stores, employees constantly pulling out ones that have recently died, largely because of overcrowding.