Need expert opinions. how often is it safe to have a ct scan for kidney stones?

Only when needed: You should only have a ct scan when the benefit to you at that moment is greater than the risk from the radiation. Hard to measure, but, you should not get one "just to see where the stones are". You should try to always use techniques to limit radiation dose, dual energy ct can provide insight as to what kind of kidney stones there are to maximize treatment, and consider MRI (no radiation).