I would say you could keep 5 minnows in a 20-25L tank if you had a filter, that would be about the smallest because as Sam says, they like to swim about. In your little round tank it might make a bit of a whirlpool if you get some current, so it's more suited to a slow moving fish like a fighter that could do without a filter if you changed some of the water every day.
You could make a nice little underwater garden in your bowl though, and wouldn't need a filter. Could add a couple of shrimp or snails.
Re how to set up a cheap minnow tank, this is what I have found, if you bought it all new:
30L tank from HFF $31
gravel $14
minnows $20
small internal filter $25
light fitting - $25-70 depending on type
a few plants - $10
you could get most of those things about half of that cost if you didn't want them new