Let \( A=\{2,3,6,7\} \) and \( B=\{4,5,6,8\} \). Let \( R \) be a relation defined on \( A\times B \) by \( (a_{1},b_{1})\,R\,(a_{2},b_{2}) \) if and only if \( a_{1}+a_{2}=b_{1}+b_{2} \). Then the number of elements in \( R \) is ________.
