what's the best highest attribute point my sorcerer should have in charisma, so i can use the rest of the points in the other attributes?

No such thing. You need at least an 18 to cast all the spells you are likely to get in normal mode, 19 if you plan on doing HoF.

However, your prime spellcasting stat bonus modifies the save against your spells. The higher your charisma as a sorceror, the harder it is to resist your spells. So if you have a 24 Charisma, all of your targets' saves are at 17+spell level, whereas if you only have an 18 they are 14+spell level.

Plus high charisma gives more spells. You should always raise your charisma with your sorcerer