What is the minimum number of coplanner vectors of different magnitude which can give zero resultant?

A singe vector with a magnitude of zero produces a zero resultant. -- Two vectors with equal magnitudes and opposite directions produce a zero resultant. you'll need at least three. Think of them as being connected. To have a zero resultant, putting the vectors together head to tail should form a closed shape. The first vector can be in any direction. The second vector starts where the first ended, and extends in a different plane. The last vector starts from where the second ended and extends to the beginning of the first vector. The three end up making a triangle, which gives you a zero resultant