Numbers for a shoaling species is usually a matter of making sure there is sufficient space for a decent sized group, and the more there are the better the fish will be health-wise. This holds across the board for all such species because the expectation is programmed into the fish's DNA. Provide what the fish "expect" and hopefully all goes well, and it usually does with mnost of the shoaling species (all characins, cyprinids, atherinids, some catfish, etc.).
There are however a few species where the number is even more critical. Tiger Barb is one, and Serpae Tetra is another. A group of at least 12-15 in a 30g (29g basic) tank is absolute minimum. No other upper level fish (unless a significantly larger tank); substrate fish (cories, some other pleco-types) are usually OK with these, but the TB or the ST must be the only upper fish, and in this number. This usually keeps them disciplined, though again individual fish do not read the scientific literature on how they are meant to behave and may decide differently.
The second thing I need to mention here is about adding more to an existing group. It is for good scientific-based evidence that I and others always say to put the entire intended group together at the same time in the display tank; this too holds for all shoaling species. The hierarchy in those species where this is relevant will become established fairly quickly; in all cases the fish will settle in faster and with less stress meaning less trouble going forward the more of them there are. With numbers like 12-15 for the TB and ST there is generally a peaceful establishment within the group. If you start out with fewer than the entire intended group, there is usually trouble not far down the road. Adding more to such an "established" group may or may not work; newcomers can be torn to shreds within days sometimes.
The third thing to keep in mind is that both the species named above have been commercially raised for decades. This inevitably causes problems for the fish, and increased aggression is a common one, along with a weakening in general. So you have a couple of strikes against this working right off the bat.
