I think the absolute minimum they'd cover would be up to the end of volume 4, when Lorelei perform.
More likely, with 12 episodes, they could cover most of what's in the first 6 volumes. Going further and getting through Shiho's classical music backstory seems like too much and it's not as though the anime would need it. Whereas I think doing the fireworks festival would be a reasonably satisfying way to end the anime.
That would leave the wager established, yet unresolved, but I can't see this getting a 2nd season—I don't feel like episode 1 had the alchemy for that to happen. It ought to be a solid yuri anime anyway.