You should be careful changing that number - the likelihood of deleting a false positive is very high.
However if you insist, the code to delete the spam email lives in the .procmailrc files for each user, and is created by the template file:
/etc/e-smith/template-user/.procmail/30_spamfilter_sort
To change this you should copy it into (creating the directory if necessary):
/etc/e-smith/template-user-custom/.procmail/30_spamfilter_sort
and then edit the file.
You then need to rebuild all the .procmail files for each user but I am not sure how to do this - perhaps someone who knows can fill in the last stage?