Each classification society will issue only one "certificate of class" to a ship.

Each ship can have more than one "Certificate of Class"

Confused ? Don't worry !! I know the two statements looks contradicting to each other but these are not. Let me explain.

Each ship need to be designed, built and maintained as per the rules of the classification society aprroved by the flag state of the ship. 

When the ship complies with the rules of the classification society that the ship owner has choosen, the classification society will issue the "certificate of class" to the ship.

So the classification society will issue only one "certificate of class" and there is no reason or need for a second "certificate of class" to be issued.

But few ship have dual class. For example, all the ships of "The shipping corportation of India" used to have dual class. Out of two, one classfication society was "Indian Registrar of shipping". Indian ships were required to have IRS as its classification society but as IRS was then not a member of IACS, ships were forced to have a second classification society which was member of IACS.

So in this case, these ships would have certificate of class from each classification society. 

Off course, In 2010 when IRS became the member of IACS, Indian ships were no more required to have Dual class.

But if a ship has only one classification society, it will have only one "certificate of Class".