Answer
To have a full octet in valence shell, a sulfur atom must gain 2 more electrons.
Work Step by Step
Sulfur $(S)$ belongs to group 6A in the periodic table. As the number of valence electrons of an element equals its group number, a sulfur atom has 6 valence electrons.
To reach a full octet in valence shell means to have 8 valence electrons.
Therefore, to have a full octet in valence shell, a sulfur atom must gain 2 more electrons.
