A ---> B is a conditional statement, meaning if A, then B. From this, we actually cannot conclude either of your statements, because we don't know if A exists.
However, once we are told that A exists, we can only conclude B --- some --- A. To conclude B --- most --- A would require more information, because we do not know about the overall population of B.
(I hope not to cause confusion with "A exists." In the example you included, we are told that there is a security council of 5 nations, so we know that these countries do indeed exist.)
