(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.
The name Mammillaria comes from the Latin 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.
In general, it will bloom easily.
Recommended temperature zone: USDA: 9-11
Frost tolerance: Hardy to 20°F (-7°C)
Minimum avg. temperature: 50°F (10°C)
Sun exposure: Full sun to light shade
Growth habits: Solitary, up to 4” tall (10 cm), 6” wide (15 cm)
Watering needs: Water sparingly, needs good drainage
Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz