For instance I want to kill an NPC via mixing poison in a soup and giving it to the NPC, assuming that the they will drink all the soup. But my poison's damage isn't enough to kill them. If I put more than one dose of poison could I make the soup kill the NPC or is one dose same as five doses of poison?
The following is a direct quote of Purple Monkey's answer to the following question:
Game effects, not just magical effects, of the same name don't stack. See the DMG errata:
Combining Game effects (p.252) This is a new subsection at the end of the “Combat” section:
Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap...
So only one dose (1d4 damage) of the poison would apply.
The same applies to the soup, because multiple poisons of the same name would be effecting the soup, only one can actually apply.
However, if you were mixing together different poisons then they would all stack because they do not share a name.