https://war-brokers.fandom.com/wiki/Halloween_2024
1 antidote = 1 prize
Once you have gotten the prize, you cannot get any more prizes for 1 antidote.
2 antidotes = 1 prize
You have now collected two different prizes.
Assume you have gotten 0 prizes so far. If you get 5 antidotes then you start by getting the prize for 1 antidote. You go up in the order as you get each prize. Prizes begin to require more and more antidotes. Once you have gotten all the prizes (up to needing 7 antidotes) then you no longer get prizes from the antidotes.
The prizes are predetermined and are not random like flags or whatever. You should expect things like the hats in the shop (that cost 100, 200 300... 700 coins).