First, CMS (§14.76):
1. For books/articles with up to six authors/editors, list them all.
2. For books/article with more than six authors/editors, list the first three and then et al.
SBLHS2 handles things a bit differently (§6.2.3):
1. All names are generally listed in the bibliography—but…
2. Using et al. following the first author's name is permissible.
With some of these archaeological articles that have forty-two or more authors, I think I'll go with option 2 if I'm following SBLHS2!
Table of contents for copyediting stuff.
Updated as an afterthought:
One publisher I work for has a general rule of thumb that if the list of authors is more than ten, they use et al. I think that's a handy one and I use it when following SBLHS2, regardless of the publisher, unless their style sheet says otherwise.