indeed, yes, an interesting question. I think it depends what you mean. The Codex Calixtinus lists only 3 shrines on the
Camino Frances: Santo Domingo de la Calzada, Sahagún (= St Facundus, though there was also the shrine of a certain Primitivus), Isidore at León. I suspect that Santiago will have wanted to prevent too much competition. One that sprang up later that comes to mind is San Juan de Ortega, a follower of Sto Domingo. A cult also grew up around the statue of the Virgin at Cebreiro, but this is not 'relics' as such. If you include the Aragon route under '
Camino Frances', then add San Juan de la Peña. Other places (Sta Cristina at the Somport, for example) acquired relics over the course of the Middle Ages, as you had to have relics to have any street cred. How many of these are still around, I wouldn't like to say.
The other major shrines in N Spain are not on the
Camino Frances: Oviedo, the True Cross at Liebana (celebrates its Holy Year this year), Covadonga and Zaragoza (again, not relics as such).
If you include all places where an image of some sort is venerated, then there must be hundreds in Galicia alone, but I'm afraid I don't know of any book on the subject.