Check at the pilgrim office in SJPP when you arrive. Last year I walked the Valcarlos Route, against the advice of the Pilgrim Office. At that time they were encouraging all pilgrims to go over the mountain on the Route Napoleon, I guess in hindsight that was to keep them all together for safety. The Pilgrim Office told me that if I walked the Valcarlos Route, I'd be the only pilgrim there, and they were right. I only saw one other pilgrim, and he turned back!
If you do choose to walk the Valcarlos Route, please stay on the main road. It's more tiring on the feet, and apparently steeper going up the Ibaneta Pass, but the off-road paths are now very overgrown and lonely due to lack of walkers.