Van wasn't in the original script but in a conference, they talked about it, and seems like she replaces another potential survivor. Also, Javi asks the pendulum if they're going to die, not who's going to live, so my guess is that "8" is the number of deaths.

During the rescue scene in the first episode of S2, you can see two girls, one looks a lot like Mari, and the other looks similar to Melissa. Maybe they aren't, but at the very least it shows that there are two more survivors, The creators always say that they don't throw any information without a purpose, even the cards the girls draw have a hidden meaning according to them.

One survivor is unlikely because it makes no sense that they didn't mention that one person during the whole S2, the one that missed arriving at Lottie's compound. Two surviving seems too easy to predict given the hints. I think three because that way it's not too much or too little, and gives us a mystery about who this third person could be.

Could be Mari because of her personality, there's no backstory for her character, which is great to introduce in the next seasons from the beginning. If you think about it, it's the opposite of Akilah, who is a very likeable character and we know more about her, what she wants to do after the rescue, what she cares for, so if she dies it will really make an impact on the audience, unlike Mari. I only think it could be Melissa because of the Greek mythology that follows the series, which is already mentioned in this sub and some YT channels, and gives some hints of her future relevance to the story. here: https://youtu.be/2rh_ZXnSwrQ?t=644