We stayed in various campsites in that area, I don't think they fill up, but wild camping is permitted in Spain, and pretty much every town seems to
have an (unofficial) car park where you can camp, you can find the places by seeing a cluster of vans. On the whole the campsites were pretty cramped, especially the cheaper ones and the more expensive ones were good, but er, expensive....
One of the best places we wild camped at was at a tiny village called Playa de Cue - hopefully link below works
https://www.google.co.uk/maps/place/Playa+de+Cu%C3%A9/@43.4164997,-4.7302452,1347m/data=!3m1!1e3!4m13!1m7!3m6!1s0xd49497e3725aa19:0xdedb8dc4c9b0a589!2sCalle+Portugal,+39007+Santander,+Cantabria,+Spain!3b1!8m2!3d43.4614289!4d-3.8254254!3m4!1s0x0:0xce8821ee5e6f3b3a!8m2!3d43.4165006!4d-4.7302446
Nice beach, about 15 campervans of various types wild camping when we were there, and easy walk to pretty village with nice cafe.
We were camped between a honeymooning Dutch couple and a professional Italian Diablo/Juggler who gave my kids a few lessons, and ever since taking the Diablos has been an essential part of camping...