Let's assume that both forces are apllied to the same point. In another case they would cause rotation of a body and it would be unclear what the net force is.
Forces are vectors. Two vectors a and b with the same starting point are always lie in the same two dimensional plane. The magnitude of their sum is the square root of the dot product:
`sqrt((a + b)(a + b)) = sqrt(|a|^2 + |b|^2 + 2|a|*|b|*cos(c)),`
where c is the angle between a and b.
The maximum value of this magnitude is reached when cos(c) = 1, this means the vectors have the same direction. The value is actually |a| + |b| = 12 N + 16 N = 28 N.
The minimum is reached when cos(c) = -1, when the vectors have opposite directions. And this minimum value is ||a| - |b|| = 16 N - 12 N = 4 N.