Have you done any research at all yet? If not, go spend a few hours reading about them as we're not here to do it for you.
From personal experience, a 2.5*1*1 ft tank will only take one pair, as they are quite territorial. The best thing to do is to buy a pair (this does not mean a random male and female, but a "couple", two fish that are "partners"), or 3m 3f juveniles and keep only the best pair once they pair up.