Too many fish?

Hi group.

I'm very new to keeping fish and I'd like to check something out that has
been said to me about the number of fish I have in my tank. The tank is 24"
x 15" x 12"

I bought the tank off a guy just short of 3 months ago. The fish that came
with it were 2 black moors, a fantail goldfish, a golden orf, a black widow
and a male fighting fish. Since then I have added...

5 zebra danio's
3 mountain minnows
1 albino red tail shark
2 guppies
2 platys

I have been told that there is way too many fish in a tank that size and it
will stress the fish out. Is this a fair comment? All the fish get along
fine. One of the guppies picks on the smaller guppy, and he hides a lot and
the danio's are always chasing each other around. Other than that it seems
a peaceful tank. They all leave each other alone and seem happy enough.

I really wanted to add a shoal of 10 neon tetras to it but now i'm not too
sure what to do.

Obviously if the tank *is* over-crowded then i'll give the neon's a miss.

Saying that, I saw an advert for a same sized tank for sale that had 47 fish
in it. Didn't say what fish they were tho'.

I'd appreciate any advice