What sexes do you want? If you want males and females, go for a trio first. You will definitely have fry, and that will leave you room to keep a couple. If you're going to have fry, though, you need to plan ahead and think were they will go, and where you'll grow them up, and where they'll go when they are grown up. Females can produce fry once a month, and you can quickly get overrun.
If you want all females, start out with 2-3. As mentioned, they will also have fry, even without a male, and fill the tank up quickly.
If you want all males, 5-6 will work best. 5 seems to be the magic number to spread out aggression enough.