Q. For ACS citation style, how many authors do I need to list in the reference list before I can cut off and say et al.?
Or would it be like Author A, Author B, ... Author Z?
Guidance on the ACS style can be found in The ACS Style Guide, 3rd Ed. Although we have this guide in print on reserve at the library, coincidentally we also just posted links to the online version which can be viewed on- and off-campus. Since your question came in before we posted these links, I'll tell you exactly where to look.
You can access The ACS Style Guide, 3rd Ed. either on our How to Write a Reference Page, on our Databases A-Z List, on our Chemistry, Biochemistry and Pharmaceutical Chemistry Guide, and on our eBooks page. Here are those links:
The chapter you want is in the "References" section in Part 2, Chapter 14. Page 289 discusses citing multiple authors in text, while page 291 discusses the reference list.
I hope this is helpful. Please let us know if you have more questions.