keycaps

on almost every model, the answer will be, "You don't. They are not replaceable."

what Mac by model-year and scrensize and intel or M1?

In theory Apple or at least an Apple Authorized Service Provider should be able to replace the individual key caps for the USB-C Apple laptops. If your laptop uses a scissor mechanism and it is damaged, I'm not sure whether an AASP has access to replacement scissor mechanisms. If you have a laptop with the Butterfly mechanism and the butterfly mechanism is damaged, then you will need to replace the whole Top Case/Keyboard Assembly since the butterfly mechanism is not replaceable.

Places do sell replacement key caps and scissor mechanisms for Apple keyboards, but keep in mind that there are multiple versions of these items even for the same exact model year which are not physically interchangeable. I also know that the USB-C laptops that some of the keys may have different lettering of the special keys and the color of the black key cap may have slight differences which can stand out against the original keys. Also, while you can get replacement key caps for the butterfly keyboards, you cannot get a replacement butterfly mechanism. It is really easy to damage both the key cap and the scissor/butterfly mechanism if the key cap is not put on properly (or removed properly). The key caps are really thin and fragile and easily damaged.

I don't have any recommendations for purchasing key caps. I tend to pull them from scrapped parts when available.

A2706 looks up to an error message.