As far as I know there are the four major ones:
WBC:- World Boxing Council
WBA:- World Boxing Association
WBO:- World Boxing Organisation
IBF:- Intrnational Boxing Federation

and some minor ones:

IBO:- International Boxing Organisation
IBA:- International Boxing Association
IBC:- International Boxing Council
WBU:- World Boxing Union
WBF:- World Boxing Federation

Thats 9 international ones!! Any I missed out?

Also which belt is the most credible, WBC because its oldest?
Which is least credible?

Has anybody held more than 6 (RJJ:- WBC,IBF,WBA,IBO,WBF,IBA [also NBA])?

Is there somewhere to find out the history of these organisations?