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Why did You-Know-Who have only 7 You-Know-Whats?

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?