Damage distribution while blocking many creatures at once
510.1c A blocked creature assigns its combat damage to the creatures blocking it. If no creatures are currently blocking it (if, for
   example, they were destroyed or removed from combat), it assigns no
   combat damage. If exactly one creature is blocking it, it assigns all
   its combat damage to that creature. If two or more creatures are
   blocking it, it assigns its combat damage to those creatures according
   to the damage assignment order announced for it. This may allow the
   blocked creature to divide its combat damage. However, it can’t assign
   combat damage to a creature that’s blocking it unless, when combat
   damage assignments are complete, each creature that precedes that
   blocking creature in its order is assigned lethal damage. When
   checking for assigned lethal damage, take into account damage already
   marked on the creature and damage from other creatures that’s being
   assigned during the same combat damage step, but not any abilities or
   effects that might change the amount of damage that’s actually dealt.
   An amount of damage that’s greater than a creature’s lethal damage may
   be assigned to it.


If the defending creature has first strike and deathtouch, you can deal only 1 damage to each creature, as deathtouch make it lethal.

UPDATE: Thanks to tengfred to comment the key rule for this:


  702.2c Any nonzero amount of combat damage assigned to a creature by a source with deathtouch is considered to be lethal damage for the purposes of determining if a proposed combat damage assignment is valid, regardless of that creature’s toughness.