@Gopal Deshmuk Don’t forget the basics. In sign magnitude representation, if n bits are used for representation then out of n bits, 1 bit (MSB) represents sign and (n-1) bits will denote magnitude.
So, while doing any operation (Addition, Subtraction, Multiplication, etc.) on n bit values, the magnitude may overflow i.e., it may require one more bit than normally. Hence to avoid overflow magnitude will be given one more bit than usual i.e., $(n-1)+1 = n$ bits and 1 bit will be required for sign representation.
Thus, total bits required = ${\color{Red} ((n-1)+1 )} + {\color{DarkBlue} 1}$ $= {\color{Magenta} (n+1)}$ bits