User blog comment:EmpyreanSmoke/Supernatural Q&A/@comment-79.115.73.90-20150811110255/@comment-24834479-20150811232448

Yes, everyone that goes to hell eventually becomes a demon, so there are millions to billions of demons in hell. Crowley is very powerful, so it is likely that he uses some of these souls to boost him up when he needs it. We don't have him become too powerful off the souls for the sake of the plot, but he could use them more for his advantage if he wanted to.