Pasta only takes on seasoning while it cooks. Whatever comes later sits on the outside. That is why the cooking water decides whether a dish tastes rounded or flat.
The rule of thumb and what is behind it
Ten grams of salt per litre of water and a hundred grams of pasta. That is roughly a heaped teaspoon. The amount sounds like a lot, but only a fraction ends up in the pasta -- the rest stays in the water.
The pasta water is an ingredient
The dissolved starch in the cooking water binds fat and liquid in the sauce. One ladle turns a separating sauce into a creamy one, with no cream and no flour. That is why Italian kitchens never pour all of it away.
- Add salt only once the water boils, otherwise it takes longer.
- No oil in the water -- it only stops the sauce from clinging.
- Drain a minute before the packet says and finish cooking in the sauce.
- Always keep a cup of the cooking water.
Follow those four points and most sauces need neither cream nor butter to come together.