(This plant comes in a 3.5 inch pot. You will receive a very similar plant to the one in the picture. It may or may not be blooming at the time of your purchase. The plant is shipped in its pot to prevent any damage to the roots).
There are around 300 species of Mammillarias. Most are native to Mexico, some are native from southwestern USA, Caribbean, Central America, or even a couple in northern South America. In their range you can find Mammillarias from sea level to high elevations. They also vary in size from miniature 1 inch in diameter (2.5 cm), to columnar types 1 foot tall (30 cm).
The name Mammillaria comes from the Latine for "nipple", because the areolas (structures carrying the spines) are carried by nipple like structures instead of being organized in ribs like many other cacti. Another significant difference is that the flowers are not coming from the areola, but from the area at the juncture of 2 tubercles.
As could be expected from so many species, their growing requirements are quite varied. Many species are easy and fast to grow and to get to bloom while young, this is why they are very popular Some are quite hardy while others will rot easily if kept too cold. All of them require excellent drainage. This is more easily accomplished by using a good cactus soil mixture, and using a shallow pot.
In general the Mammillarias bloom easily, but many species need a rest period in winter during which they are kept cool in order to bloom. Furthermore flowers form a ring on tissue grown the previous growing season, so it is important to get them to grow every year, which means regular watering and fertilizing during their growing period.
Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz