It would help if you opted for a sum insured which must be adequate to cover all healthcare expenses you or your senior citizen parents may incur during a year.
If your parents are already suffering from pre-existing illnesses or if they have a history of medical complications, you should go for a high sum insured, i.e., ₹10 lakhs or more. You should also consider your parents' lifestyle and personal habits while choosing the sum insured for their senior citizen health insurance policy.