Buen camino, Hakunkim! it will be quite cold, and days will be short. I am not sure many albergues will be open, but you should be able to find private accomodation in pensiones, hostales, hoteles, rural houses and things like that (definitely, book in advance to avoid nasty surprises)

It will most likely be cold and wet between Sevilla and Baños de Montemayor, but after that it will be very cold, most likely below zero every morning, all the way to Santiago. It will be a very lonely camino, you must know that too, but the landscape is fantastic.

There is a blog where every month they public a document with all accommodation which are opened. It's the most current information about "Via de la Plata", so in December, check this blog.

https://viadelaplataelcamino.blogspot.com/2020/01/guia-albergues-via-plata-sanabres.html