Marines are alot more work and extra cost than freshwater aquariums.
as with the number of fish, I think you have to take in mind the type of fish, Plants etc., number of water changes, water quality, bla bla bla .......
So if you do everything right in a tropical tank and do the best for you little pets they should be fine.
Thinking back I probably over stocked my fist tank which was 23L with only an under gravel filter and plastic plants :slfg: