What adjustments would be needed to prepare these as single serving size

The recipe should work if you just roll smaller pieces of dough into the size of galette you would like to serve. The baking time would be reduced for the smaller size. Depending on how small you make them, it might only take 15 to 20 minutes. Just watch for the beautiful brown color on the crust (lift it slightly to see the bottom if you can) and check the doneness of the apples.