The statistics from the pilgrim's office in Santiago seem to show that only about 4%-5% of pilgrims didn't walk for religious/spiritual reasons (i don't remember the exact figures but you can find them easily online). I think they should break the groups apart: non-religious and non-spiritual, spiritual, and religious. Just because I'm curius.
BTW, it seems some people also say it was religious/spiritual because they don't want to get a "fake" compostela (you get a different paper if you didn't walk for religious or spiritual reasons). I have no problem with my "fake" compostela! It's just as pretty, and they've recently changed it to make it look nicer. It's also written in Latin now, like the "real" one.
(Post edit: Made myself look at this year's statistics. It seems like I was wrong and they do break the groups apart. Also the numbers are changing...
http://peregrinossantiago.es/eng/pilgrims-office/statistics/ See under Peregrinos por Motivacion)
My own experience at least shows that most people will be at least somewhat spiritual. However "spiritual" is difficult to define sometimes, and "finding yourself" isn't always a mystical experience. I got the non-religious compostela because I didn't think I had a "spiritual" experience. But I'm a skeptic
From Sarria on you'll meet many more religious pilgrims, or at least, that's what I've seen.
I hope you have an amazing, emotional experience! Buen camino Mike.