Any idea how much charisma to pass most check???
No matter how much you stack Charisma there's always going to be an appreciable chance of failure. To take chance (almost) entirely off the table you'll want to invest in spells that improve your chances by granting bonuses and things like advantage.
Honestly, the best thing to do is just make sure one of your companions has Guidance. Shadowheart already comes with it. It's an easy +1 to +4 to a check, depending on the roll. Which can be huge. Look for spells like Enhance Ability too. Friends and other Charm effects can come in clutch, but it has a drawback that once it wears off the NPC will get mad, so only use those on NPCs you never intend to meet again. These spells can be on your companions too, you don't need to build around them. They just need to be nearby to cast them on you.
If you have a decent spread of magical bonuses and proficiency in Persuasion you should be fine even with Charisma as low as 10. Just pick and choose where to use your social spells, since you might not have the slots for it on every check.
