VERY hidden tidbit on page 104 of the DMG... There is no limit to the number of cohorts a character can have. I don't *understand* this idea, considering how the number of followers you can have is very set, but it's right there, buried but plainly and deliberately written, 2nd paragraph in the Cohorts section.
Feats that increase leadership have already been given here...
In my particular campaign, we're given the option by the DM whether or not we wish our charisma affecting items to increase our leadership score. The crux of the choice being that if we ever lose our item, it will affect our standing with followers and cohorts by more than just the mathematical loss of leadership score, akin to betraying a friend with a "false" image of you.