Pretty much nil. Psychedelics dont release serotonin, different mechanism to mdma. Mind you does mdma and ssri really cause serotonin syndrome? Or is it just some self diagnosed one in a billion reaction someone had and so, like these stories do, instead of one in a billion its now a certainty?
9 people a year die from eating satsumas but i wouldnt panic the next time you are handed one. Serotonin syndrome seems to be something people reach for when thet cant handle their high - i dont think ive ever heard of a genuine case diagnosed by someone with any medical knowledge apart from reading reddit.