Spoiler for those who haven't read Half Blood Prince
In 7th part of Harry Potter it was said that to get Horcrux, you need to murder a person. Voldemort has/had only seven Horcruxes. Why? I mean, he killed thousands of people, why only seven? Did he have to cast a spell while killing them if he wanted a new Horcrux? Are there some other requirements or limits to creating a Horcrux?