I have read the three mangas and I honestly don't know how it will be stretched to 12 episodes

well lots of fillers and maybe the movies they shoot in the movie club can have a detailed plotline of its own. Basically a story within a story.

i am kinda hoping that they include the story of other couples too

eu estou torcendo para 2 season desse anime

I think they will def include all three couples' stories in this so that will def help (or at least, I hope so, cuz ima be pissed if they don't animate Ichikawa's story). But I don't think they are gonna include anything from the sequel story because it jumps into the future a bit.

This is just an example, but I'm pretty sure the first season of Given only included the first 4 volumes, so it may not seem like it was a lot, but if they do it right, it won't seem super stretched out. Or at least, I hope so.