I would definetly say NONE! Sorry, for guppies, if you plan on having males and females
Which you definetly would do this since the males are the colorful ones, and you have to have females with males, unless you want one male IMO.
So, anyway, guppies breed alot so they need at least 10g of water IME for them to thrive. I would put 1 male and 3 females in a 10g tank.
Also, guppies require a heater and filtration.
I would look in to bettas, even though they prefer to have a heater, also.
Good Luck!
PS: Please, please dont put a goldfish in there!