"Accompanying him were the Twelve and some women who had been cured of evil spirits and infirmities, Mary, called Magdalene, from whom seven demons has gone out, Joanna, Susanna, and many others ..." (Luke 8:1-3)
Apparently Jesus had cured Mary Magdalene of illness - that's what is often meant by "demons." The number indicated the seriousness of the sickness. Some later writers, mistakenly thinking demons referred to sin, identified Mary Magdalene as a sinful woman.
- Little White Book, Diocese of Saginaw