According to what I have gathered on the James Cameron's Titanic about the movie extras:
- There were a total of 18,000 extras employed for the film.
- 150 stayed the entire six months (October of 1996 to March of 1997) for shooting on the RMS Titanic set, they were the Core Extras.
- 500 of them worked as the people in the water that were there for two weeks (that's how long the open water sequences took to shoot).
- 2,000 extras were hired for three day shoots.