Note that even if you're not intending to use any torches for light, if you want to use shambler altars, you'll need to bring at least one torch.

Bringing twice the average amount is a good rule of thumb.

For example, using N=100 tiles, P = 5% chance of a food tile, the result is an average of 20 food needed, but the 95% interval is at 40 food needed. The binomial distribution with typical Darkest Dungeon parameters for food has a long tail. It's quite often not enough, even in dungeons that normally drop a high amount of food from Curios (Cove, Weald). I personally always bring the maximum amount of food. I find that in long dungeons, you don't really need more, as you don't need to snuff the torch for extra loot, you'll have plenty without doing that (and save the 75 gold). Typically, a short dungeon has ~4 battles, a medium ~8, and a long about 12, so you would want 12, 24, and 24 torches (counting the firewood as 4). So ideally you want 4 torches per battle beyond the first. (You should snuff out your torch completely before looting any curio and before a battle ends to increase the loot gained). I find that when playing with Torches and Food, bring lots of both if you want to benefit from Curios more. Obviously, you'll always need Food and Shovels.

It depends on what you want out of a dungeon. There's no simple, straightforward answer.

Darkest dungeon long ruins provisions