 This name that Yubaba gives to Chihiro is the    Japanese pronunciation (AKA "on" reading) of first kanji in her    name (chi). Since kanji is a system of ideograms as well as    phonograms (like the Chinese "Hanzi" system of writing), removing    the second half of her name requires the first half of her name to    be pronounced "Sen" and not "Chi", although the meaning of    "thousand" remains. By having her original name removed, her    identity is also removed, and her new name is essentially a    number.