Being the sum of a person's personality and charm, the Charisma of a Character affects his or her ability to persuade others, and also determines the maximum number of Non-playing Characters who will follow him or her willingly. The abbreviation for Charisma is given as "CH". As with other primary stats, there is a special bonus for gaining a Charisma value of 20 or higher. Your followers will not leave the party any more if the player's actions and decisions conflict with their morals.
A note in an interview mentioned that followers is determined by CH/4, plus 2. 
- ↑ Arcanum: of Steamworks and Magick Obscura Frequently Asked Questions. Terra Arcanum. 2010-06-25. point #25 from Wartoy on 2/7/2001