Hi Richard,
During my Caminos I have met many solitary individuals who were alone, myself included. From experience, I can assure you it is entirely possible to be alone in an albergue - even if there are many other people around!
Sometimes, while walking, you fall in with a group. They walk the same distance, at about the same pace and you see them daily at the end. Other times, you find a different rythym and path and can find yourself entirely alone. During my Caminos, I found that there were both pilgrims with whom I needed/wanted to walk and others with whom I needed/wanted to part.
The Camino is so personal and each pilgrim's motivation is his own. All pilgrims will respect that. I would only offer one small piece of advice. Try not to be too "rigid" in saying how the Camino will be..."I shall do this or that", "It will be this or that". Be open to the experiences that the Camino will present to you - they will be many and varied.
As to the credencial, as others have mentioned, if you are beginning in Sarria (or thereabouts) and only walking the last 100km then it is a very good idea to have at least two stamps per day from various places along the Way. If you begin father back, it is not quite so necesary to have two per day, but still a good idea to have some from many towns along the Way. In 2008 I began my Camino in Moratinos and got as many sellos as possible so I would not have blank spaces in my credencial. It was a beautiful one that I had gotten in Paris and I wanted it to be a complete momento. Silly, but different motivation nonetheless!
Buen Camino,