bibtex-autokey-titlewords

bibtex-autokey-titlewords is a variable defined in `bibtex.el'.
Its value is

5

Documentation:
Number of title words to use for the automatically generated reference key.
If this is set to anything but a number, all title words are used.
Possibly more words from the title are used according to
`bibtex-autokey-titlewords-stretch'.
See `bibtex-generate-autokey' for details.

You can customize this variable.