Thanks !It depends where you are starting. The crowds will be dwindling for France and Pamplona starts. Crowds will still be huge for the last 100km. September in general has seen strong growth in the number of pilgrims from those who have avoided July and August, so booking is advisable if you are nervous. In general you cannot book junta and parochial albergues, and only about half the private albergues take reservations.
This site has good information on who takes reservations:
http://caminodesantiago.consumer.es/albergues/#camino-frances
We walked mid August to mid September last year. The only place we had trouble with a room was Zubiri (just before Pamplona). We grabbed a taxi to Pamplona, then the next morning took a taxi back to Zubiri and started walking again. The weather during our camino was awesome. Only sprinkled two days toward the end and only used a light fleece jacket 3 mornings at the end. Wonderful time to walk!! Buen Camino!!