When asked to find ALL unit vectors perpendicular to the plane of the triangle ABC (R3), how many results should I get?

Answer 1

Any surface has an "upward" and "downward" normal vector, the latter being exactly opposite in direction to the former

So if (say) the "upward" normal points "up and to the left" then the "downward" normal is parallel, lying on the same line, pointing "down and to the right"