When to see a neurologist if your hands or fingers get numb sometimes?

If it persist. Transient localized numbness lasting less than 2-3 minutes will not find a good cause---numbness which persist with other symptoms like weakness, pain, cerebral symptoms, spinal symptoms should see neuro.
Sooner is better. The sooner you can figure out what's going on the sooner you can treat it so it doesn't get worse and potentially cause permanent damage.