The number of equation/s that are needed in order to solve the problem is equal to the number of the varying variables involved. In this regard, we would say that to solve for 1 unknown variable, only 1 equation is needed.
For number of variables greater than 1, as stated above, the number of equations should be equal to that number. However, it has to be noted that the equations should be independent and not the multiplicative of another.
For the concept of multiplicity,
x + y = 0 and 2x + 2y = 0 are just the same equations.
