Some products are named "C20" or similar to imply that they contain 20% vitamin C. Most often these products contain synthetic derivatives of vitamin C (ascorbic acid) such as ascorbyl tetraisopalmitate.

Ascorbyl tetraisopalmitate contains 15% ascorbate by weight. Even if a product contained 20% ascorbyl tetraisopalmitate it would only contain 3% actual ascorbate which could act in the body as vitamin C.

Other synthetic vitamin C derivatives, sodium ascorbyl phosphate and magnesium ascorbyl phosphate, have been reported in independent scientific studies to minimally penetrate into the skin.

To have a biological effect any synthetic derivative of vitamin C has to penetrate into the skin and be turned into pure vitamin C, ascorbic acid, which is the only molecule the body recognises as vitamin C.

The reason most cosmetic companies don't use ascorbic acid? They can't stabilise it well enough. Just as most restaurants can't put together and serve three Michelin star quality meals, most skincare companies can't put together and market stable formulations of highly unstable ascorbic acid. It's much easier to use more stable synthetic derivatives of ascorbic acid.