one... a betta or a guppy.
tiger barbs need to be in groups of 5 or more and get almost 3inches. you'll need at LEAST a 10g for them.
danios gt about 2 inches and again need at least a group of 5. they're VERY active fish that need room to swim. yet again, 10g.
there is no such thing as a small goldfish, only YOUNG goldfish. even the smallest types of goldfish (fancy) get up to 10" and the more commonly thought of ones (thin bodied) get up to 18". not only do goldfish get very big, they're also poop machines. you'll definitely need at least a 10g for even one goldfish.
try to imagine being the size of the fish and your house bing the size of the tank... would you want to live in there with other people? or even at all? it's basically like spending the rest of your life in your room, small tanks aren't good for much.