There's no right to remain silent when it comes to handwriting. A potential problem exists with signatures. Also, other circumstances affect writing such as age, arthritis, emotional distress, medication, intoxication, and corrective vision. It's difficult to tell handedness, gender, and age from unknown samples. With an impairment like arthritis, for example, the writing function may become more of a conscious than subconscious effort, making for a more complex case.
Problems get stickier when trying to draw personality inferences from handwriting samples.
Remember this is the realm of graphology, but it might be interesting to look at a few traits of interest to criminal justice and criminology. The courts appear to be waiting to see college psychology courses on it. There's some precedent in art therapy and projective psychological testing for graphology. There's quite a bit of divergent terminology found in court testimony. Instead, it's up to each expert to say what constitutes a sufficient number.