It depends on what you want to measure. Do you need a generalised measure of wellbeing or one that measures symptoms of ill-health? There are various levels at which you can conceptualise mental health, ranging from "living well" (Ryff's eudaemonia, or the concept of Ikigai in Japan), through things like "flourishing" or "positive mental health", through non-specific measures of mental health adversity to measures that tap specific problems like generalised anxiety disorder and depression (the two most common ones).
So first you need to conceptualise mental health for the purposes of your research, then identify a suitable instrument.