Any idea how much charisma to pass most check???
BG3 doesn't have a system that quite works like that. Skill checks are always a set difficulty number, generally between 5 and 20. Then you get to roll a die that can be 1-20 and see if you can tie the difficulty number.
Then you get to add different kinds of bonuses to you die result if you need it. Charisma adds 1 point to your roll for every 2 points above base ten. So you can get up to +3 from charisma at the start of the game.
Next you can add in 2 points if you are proficient in the type of skill you are using (persuasion, deception, etc). Some characters can even get expertise on a skill wich lets them get double points so +4 starting off.
Then you can add in some spell and special ability bonuses. There are two basic types:
Things that let you add extra points with dice: like the guidance spell (+1 to 4) and bardic inspiration(+1 to 6).
Things that give you advantage (which lets you roll twice and if either die ties the difficulty you win): Spells like Friends, Thaumaturgy, and Enhance Ability. The general philosphy is that advantage averages out to about +4 on your rolls
Your party can also get Inspiration points from doing things that are important to them, like Gale reading an important book. Using an inspiration point lets you try the whole roll again if you fail.
Plus if you ever roll a 1 you fail automatically and a 20 automatically succedes.
So the math isn't more than just "Is my charisma high enough?"
Note: the math above is based on first through third levels. As you level up, some bonuses get bigger.