Hi, Gianlucca,
I have walked from Lisbon to Oporto, and the British Confraternity has a copy of my short guide on their website, where you can download it (they ask for a donation).
http://www.csj.org.uk/guides.htm (You'll see there are several online guides available there).
I always stayed in private accommodations -- hotels or residenciales (residencial is the equivalent of pension in Spain). My guide has lists of all the possibilities I could find, with phone numbers, so that should help you out. Most of the places were not in overly touristy places, so I don't imagine you'll have trouble finding a bed, barring any special fiestas -- for instance, I know Golega has a special horse fair, but I am not sure of the days. I don't think you should worry about it, though, because as a last resort you can sleep on the floor of the Volunteer Firemen building.
I never met another peregrino on this route, but the owners of the places I stayed assured me that there were people walking, one owner had kept statistics and told me that in the spring months she usually had between 35-50 a month.
I enjoyed this route in many ways, but one thing I did not like was the asphalt. I would estimate that nearly 85% of this route is on one street or another. Usually it's a quiet rural street, but there are a few stretches along the shoulder of busy highways (but the shoulders were always ample). I'd love to hear about your experiences -- you'll find that all the libraries have free internet, so you could post along the way to let us know how it's going.
If you like Roman ruins, don't miss the villa at Rabacal (with its small museum) or the much bigger site at Conimbriga. There are lots of other nice spots to visit in towns along the way, and I have put my suggestions all in that guide. Buen camino, Laurie