Rule 509.7

If a creature is put onto the battlefield blocking, its controller chooses which attacking creature it’s blocking as it enters the battlefield (unless the effect that put it onto the battlefield specifies what it’s blocking), then the active player announces the new creature’s placement in the blocked creature’s damage assignment order. The relative order among the remaining blocking creatures is unchanged. A creature put onto the battlefield this way is “blocking” but, for the purposes of trigger events and effects, it never “blocked.” Example: Giant Spider is blocked by Canyon Minotaur. The defending player casts Flash Foliage, which creates a Saproling creature token blocking the Giant Spider. Giant Spider’s controller announces the Giant Spider’s damage assignment order as the Saproling token, then Canyon Minotaur.

Find this number in the current Rules