Say I have cast Death Ward on one creature already. Can I cast Death Ward again on an additional creature without causing the first casting of Death Ward to end?
Casting Death Ward will not end prior castings
The death ward spell does not require concentration and it states:
You touch a creature and grant it a measure of protection from death. The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point, and the spell ends. If the spell is still in effect when the target is subjected to an effect that would kill it instantaneously without dealing damage, that effect is instead negated against the target, and the spell ends.
Note that the description never says only one creature can be under the effects at a time, or that the spell ends if you cast it on another creature. Thus the spell does not end if you cast it again on another creature.
An example of a spell that does do this is the light spell which states:
[...] The spell ends if you cast it again or dismiss it as an action [...]