How many fish can live in 25 liters?

Answer 1

2-3 common goldfish can live quite comfortably in a 25gal tank for many years, though they'd be unlikely grow to full size there. It's possible to fit more in at the expense of water quality. Fancy goldfish will generally require more space especially if they live with more agile fish. Because this question is categorized under "goldfish", this doesn't apply to schooling fish which produce less waste and require less space than goldfish do.

Answer 2

The rule of thumb for aquaria is "1 inch of fish requires 1 gallon of water." Converting to SI units, that's about 2.5 cm of fish per 4 liters of water.