IIRC there's 3 main people: Cari himself, who does the writing and main renders (plus the animations prior to Dread joining in full-time); Dread, who does all the animation work since 0.6 (tho he's done some animation work as far back as 0.4); and someone whose name I forgor who does the editing and proofreading work.
There's also the modders on HS2 Discord who get commissioned whenever Cari needs some assets plus freelancers who he commissions for art (i.e. Nova's comic and the illustrations in the Anima story), music (i.e. Annie's song back in 0.8 was a commissioned piece), and voice (i.e. Isaac back in 0.4 and the Narrator back in 0.8). Not exactly permanent mainstays, but they also contribute with making Eternum what it is.
Tateshiko and a few guys back in Discord do fanwork for Eternum and are not part of the team, but IIRC they were instrumental in helping Cari replace a lot of the game's music after one of the sites where he got them changed their ToS and sent him a CnD. Not exactly ideal for people who dislike AI gen stuff, but given the circumstances it's either that or a game with a huge chunk of its music missing (and Cari don't want that as he said that the mood provided by the music was part of his storytelling so he'd rather bite the AI bullet).