There are a max of four blockers per team allowed on the track. They are called pivot, b1, b2, and b3.
The pivot has the stripe on her helmet.
B1 is the pivot's buddy at the front of the pack. She usually holds the inside line.
B2 and B3 stay at the back of the pack. B2 usually hold the inside line and B3 kind of goes wild in her blocking with the B2 keeping an eye out. When the B3 is knocked down or aside or out or something, the B2 will try to hold the opposing blocker or jammer until B3 can get back into position.
See these videos for more about the basics of roller derby.