From my past test record, any voltage above 9V will do(<=15V). When supply vary from 8.9V to 12V, the output changes less than 0.5ppm.
This is for my LTZ1000 running at 45 deg C internally, and the room temperature of around 23 deg C.
To be on the safe side, >10.5V is suffice. Therefore, two 6V battery in series will do the job.
If you supply higher voltage at 18V, the opamp and the heater control transistor will generate more heat, and no good at all. One more thing, two batteries and three batteries last the same time.
Even you have a 7V to 10V resistive divider boost, you can still running the whole circuit on 12V as Fluke people do in their 732B.