Twenty-two things. Buy them once and almost every recipe on this site is a trip to the fridge away, not a trip to the shop.
Everything else can be supermarket own brand without anybody noticing. These four you will taste.
Olive oil
One cheap bottle for cooking, one good bottle for finishing. The good one should taste peppery at the back of your throat.
Tinned tomatoes
Whole plum, never chopped. Chopped tins are the bits left over and they stay watery however long you cook them.
Flaky salt
For finishing, not for the pan. A box lasts a year and changes how everything tastes at the last second.
Parmesan, in a wedge
Grate it yourself and keep the rinds in the freezer for soups. The pre-grated tubs contain anti-caking starch.
A large roasting tin, a heavy frying pan that can go in the oven, and a pot big enough for pasta. Everything on this site is cooked in one of those three.
Garlic presses, egg slicers, anything that does one job. A sharp knife and a wooden spoon do most of it, and the knife is the only thing worth sharpening.