If a creature is dealt damage by multiple sources with deathtouch does each source need to deal at least 1 damage?

Deathtouch works on a per-source basis. A creature with deathtouch causes any amount of damage it deals — even 1 — to be considered lethal damage. However, this only applies to the damage actually dealt by that specific source. If a deathtouch source deals 0 damage to a creature, it has not triggered the deathtouch condition at all.

According to CR 702.2b, a creature with deathtouch causes any nonzero amount of damage it deals to a creature to be sufficient to destroy it. The key word is 'deals' — the source must actually assign and deal at least 1 damage for deathtouch to matter.

When multiple sources deal damage simultaneously (such as multiple blockers or multiple damage effects), each source's damage is tracked independently. A creature is destroyed if it has been dealt lethal damage (damage equal to or exceeding its toughness) OR if any amount of damage from a deathtouch source has been dealt to it (CR 704.5g).

Concrete example: A 4/4 is blocked by a 1/1 with deathtouch and a 1/1 without deathtouch. Both deal 1 damage. The 4/4 has been dealt 1 damage from a deathtouch source, so it is destroyed — even though total damage (2) is less than its toughness. But if the deathtouch blocker dealt 0 damage (e.g., due to a prevention effect), the 4/4 would NOT die from deathtouch.

So yes, each deathtouch source must individually deal at least 1 damage for its deathtouch ability to apply. A source that deals 0 damage contributes nothing toward the deathtouch destruction condition.