Answer
n+1
Work Step by Step
Successor of a set A is $A \cup ${$A$}.
A set cannot contain itself.
So, $A \notin A$.
This means adding $A \cup ${$A$} will have number of elements one more than that the elements of $A$.
So, if $A$ has $n$ elements, then successor of $A$ has $n+1$ elements.