For a set of about 20 widefield Milky Way frames, a good choice in Siril is Average Stacking with Rejection using Linear Fit Clipping.

That method is a sensible fit for widefield astro images because it handles frames that may have sky gradients and differences in illumination or orientation between exposures. In practice, that makes it a stronger “general-use” option here than a plain average when you want noise reduction but also some rejection of inconsistent pixels.

So if your goal is to stack multiple widefield night-sky shots and reduce noise, Linear Fit Clipping with average stacking is the safest recommendation from the information provided.