My Nitecore D10 runs for 55 hours (on low) on a single AA. To get 200 hours I'd carry 3 spares.

Actually I measured that using an Eneloop, with a capacity of around 2000mAh. Lithium AAs claim a capacity of 2900mAh, so using those you might only need 2 spares.

Why do you need 200 hours anyway? Do you expect to get stuck in a cave for 8 days continuous? With a battery device, you can switch it off during daylight or when you're asleep.