No.
Two is the minimum number of vectors that will sum to zero.
Two vectors: no. Three vectors: yes.
-- A singe vector with a magnitude of zero produces a zero resultant.-- Two vectors with equal magnitudes and opposite directions produce a zero resultant.
No.
Two vectors with unequal magnitudes can't add to zero, but three or more can.
Yes. Any number of vectors, two or more, can result in zero, if their magnitudes and directions are just right. One vector can result in zero only if its magnitude is zero.
equal and opposite
No.
No. The tenth vector would have to be matched by one equal and opposite vector to yield a zero resultant, or by multiple vectors in the second plain collectively yielding a zero resultant for that plane. It would be possible, for example, for 8 vectors to be on the same plane and two on a different plane to give a zero resultant.
Yes. Vectors contain both magnitude and direction. Graphically three vectors of equal magnitude added together with a zero sum would be an equilateral triangle.
It is certain that two vectors of different magnitudes cannot yield a zero resultant force.
Vectors that sum to zero are coplanar and coplanar vectors sum to zero.