If a sorcery that forces all creatures to block one is cast and that creature has deathtouch, do all creatures blocking die or only as many as it takes to kill it?
If a 2/2 creature you control with deathtouch is blocked by three creatures, it can assign combat damage to at most two of them (C.R. 510.1c). But deathtouch makes any nonzero amount of combat damage assigned to a creature lethal damage for the purposes of combat damage assignment rules (C.R. 702.2c), so that you can have the attacker assign 1 combat damage to a blocker this way even if that blocker's toughness is greater than 1. If combat damage is assigned to a blocker this way, it will be dealt that damage and then die soon after (C.R. 510.2, 510.3, 117.5, 704.5h). See also this thread.
If a sorcery that forces all creatures to block one is cast and that creature has deathtouch, do all creatures blocking die or only as many as it takes to kill it?
Neither.
The number of blocking creatures that will die is equal to the power of the attacking creature.
For example, a 4/5 Deathtouch would kill 4 blockers, because one single point of damage to each is enough to kill.
It doesn't matter if the attacking-deathtouch lives or die.
EDIT: Edited after comment 3 was posted.
The number of blocking creatures that will die is equal to the power of the attacking creature.
For example, a 4/5 Deathtouch would kill 4 blockers, because one single point of damage to each is enough to kill.
It doesn't matter if the attacking-deathtouch lives or die.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules