Yes, the more the better. Shoaling fish need groups of their own species. In their habitat they live in groups numbering hundreds in many cases. Fish denied a group will be stressed, and that causes health problems. [You can read how here: Stress and Freshwater Aquarium Fish
A recent scientific study on this issue, and the only one to date, proved that increased aggression was common when any shoaling fish was not maintained in a group. The number used for the study happens to have been five, but that does not mean the fish are "fine" with five and "bad" with four. The object of the study was to determine the effects of keeping shoaling fish in less than five, and as I said the results were increased aggression. Species normally somewhat aggressive--like Tiger Barb
, got much worse. Fish normally peaceful, like Neon Tetra
, got aggressive.
On the Tiger Barb
, these should be in larger groups to control the aggression or confine it within the group and not to other fish. Eight is usually the minimum for TB, but 12 is better. And these in their own 30g tank. As soon as other species are added, the space requirement increases. Another finding of that same study was that inadequate space also caused aggression.
Aggression is the fish's only way of dealing with frustration. If it is in an environment that is not suited to its temperament, it will be stressed and lashing out is one way of dealing with that--just as in dogs or humans.
When one acquires any fish species, one has the responsibility to provide what it needs. Fish are the way they are because that is how nature made them. We can't change this.