Tripadvisor alone refers to 119 hotels & pensions, and Gronze 10 as well as 3 albergues-- San Sebastian/Donostia is a major city of 453,000 inhabitants. However, I found the turismo staff very helpful when I was there, and they will provide you with city maps with the Camino marked on them. Another option is to stay at Monte Igueldo, on the western outskirts, which saves you a long climb in the morning.
As far as stages are concerned, they are much much less defined than on the Francese, to the point that I'm not sure if it's a useful term. I have done the del Norte more than once and, while I do not know your physical condition or conditioning, I think that you might find 5km a tad ambitious. The first week of the del Norte involves lots of ups and downs (the gronze.com site will give you elevations), and the experience of consecutive days of hiking is very different. I would suggest that every 7th to 10th day be a rest or a very short one, but you'll find your own balance.