已知向量\( \overrightarrow {a} =(mx ^{2} , -1)\),\( \overrightarrow {b} =( \dfrac {1}{mx-1} , x)(m\)是常数\()\),且\(f(x)= \dfrac {1}{ \overrightarrow {a}\cdot \overrightarrow {b}}\).
\((1)\)若\(f(x)\)是奇函数,求\(m\)的值;
\((2)\)设函数\(g(x)=f( \dfrac {x}{2} )- \dfrac {x}{2}\),讨论当实数\(m\)变化时,函数\(g(x)\)零点的个数.