Just wanted to post this, since it seems so important in all of our works (and this movie is no exception).
Chi and Sen both use the same Japanese Kanji, meaning 1000. The name 'Sen' is a play on the name 'Chihiro'
The names of many of the characters are written with kanji that provide clues to their identities. For example: Yubaba - "hot-water crone"
Zeniba - "money crone"
Kaonashi - "no face"
Boh - "young boy/child"
Kamajii - "kettle/boiler-pot old-man"
Chihiro - "thousand fathoms"
or "thousand searches"
Sen - "thousand" - 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.
Found this on imdb.com. I just thought it was pretty interesting that Chihiro was told how important it was to remember your name in order to maintain an identity. At the same time, the name given to her actually literally reduces her to a number. Pretty cool.