Not all uses will be shown a ad generally speaking.
There are regionally variations (and A/B testing(.
if you getting a 204, then you should assume that Twitch will do as instructed and play a ad to all people watching were the “rules” governing who gets shown what ad allow a ad to be played.
it’s entirely likely if you are running a LOT of ads, that Twitch ran out of eligible ads to show people [citation needed]