Baking 40 potatoes at once for a big dinner or other event sounds like an intimidating task, but it's not difficult. The work and time that go into making potatoes remains nearly the same, whether you're making four or 40. Get an empty cooler to keep them nice and toasty -- it will also make transporting them to your event easy.
Buy potatoes that are all similar in size. They need not be identical, but keeping the potatoes as close to the same size as possible ensures they will cook at the same rate, so you don't end up burning some and under-cooking others.
Pre-heat your oven to 500 degrees Fahrenheit. While you normally would only set the temperature between 325 and 400 degrees, you'll need more heat to cook such a large number of potatoes.
Run cool water in the sink and wash the potatoes well to remove any dirt that may be on them. While you can use your hands, a produce scrubber will come in handy for washing large amounts of potatoes.
Poke at least eight holes in each potato with a fork for ventilation. This will prevent them from exploding in the oven and creating a massive mess to clean up.
Mix together 2 cups of olive or vegetable oil along with 3 tbsp. of salt and 4 tsp. of pepper in a large mixing bowl.
Roll the potatoes through the mixture and place them on a baking sheet. You may need more than one baking sheet to house so many spuds.
Bake the potatoes in a hot oven for 30 minutes, then remove and flip them with tongs. Bake for an additional 30 minutes.
Check a few potatoes to see if they're done by inserting a fork into the center. If it slides in easily, they're done. If not, give them an additional 10 minutes in the oven and check again.
Remove your potatoes from oven and place them into an empty cooler to keep them warm. If the cooler remains up-opened, the potatoes can stay warm for up to 6 hours.