Rule 702.21k

During the combat damage step, if a blocking creature is blocking a creature with banding, or both a [quality] creature with “bands with other [quality]” and another [quality] creature, the active player (rather than the defending player) chooses how the blocking creature’s damage is assigned. That player can divide that creature’s combat damage as he or she chooses among any number of creatures it’s blocking. This is an exception to the procedure described in rule 510.1d.

Find this number in the current Rules