A place to sleep
I have been walking part of Via de la Plata this spring. The organization of the friend of Via de la Plata in Sevilla had a good list of Albergues. On this list, they also mention on which places you can find hostals and hotels. We experienced that their small office was not often open, and I think not at very regular times, but if you stay a day or two in Sevilla, you can find them. They were very nice to us. At least when we were there, we could only comunicate in spanish. How to find the office, you will find described elsewhere on this forum.
I think a good guidebook will be your best help. We used Alison Raju "Via de la Plata" (2005) and found it good, both for the "navigation" and for the descriptions of the villages. For every place the book can tell you about the facilities; pensions, hostals, hotels, casa rurales... For many villages you will also find both adresses and telefonnumber to these places. The book also tell you were there is nothing but the albergue, and where there is nothing at all.... Things like this change all the time, so you can never trust a book 100%, but a guidebokk will be a good help; at least our guidebook was.
Some days you will have to walk guite long distances for a place to sleep, but at least for the first 300 kilometers, I do not think you will have problems with getting a place to sleep if you telfone the day before. By then you will know how the situation is, and what you have to do.
It seems like there are more and more people walking Via de la Plata, and this is not the french route, so the places to stay are often small and can be filled up. So if you can not sleep on a sleepingmat on a "roof and floor" from time to time, I would recommend booking.
For example when you leave Sevilla, the natural place to stop the first evening is Guillena. There is a "roof and floor" and a small hostal. We had tried to get a room in the hostal on beforehand (the guidebook had the number), but no ... So "roof and floor" ...
I think you will find what you need with some preparations!
Good luck! Bjørg