These are the things you need to consider before even asking how many.

1.What you intend to do eg. Breed, Display, etc etc
2.What fish
3. How many species/variants you wish to keep

If you are going to Make a display tank then it is usually best to completely overstock you tank with males or differents species, this way there is hardly/none aggresion.
If it's going to be for breeding then buy waay to many, as you will need to cut numbers, and being for the one varient there will usually be less aggression as long as you can cut your male numbers down.

It's something that you should possibly research to see if the fish will be able to survive together happily also. Similar water parameters and the like.