What temperature should I use to keep pre-baked potatoes warm in a roaster oven for about 4 hours? There will be about 18-20 potatoes.

Once the potato is baked it should be served within 1-2 hours. Keeping it warm for four hours will result in a wrinkled skin, probably some browning under the surface and it may pick up some off flavors.

However, from a food safety issue the potatoes need to stay 135°F or hotter. That’s with a meat thermometer measuring the internal temperature. I’d try to hit 185 to 200°F. I also would not recommend baking the potato in foil as this seals in the moisture and the skin will be wet and the interior mushy. Here are some other recommendations or tips: