It varies, sometimes even within submission types at a single ACM conference. I am aware of no single set standard. Your conference should have a proper "guide for authors" or "information to authors" page, and it is often also included in the full "call for participation" or "call for papers", as well as a template in Word and/or Latex (which is also itself likely to give this information in it), and additional information about review processes, guidelines, timelines, etc. I've never seen a legitimate modern conference that omitted this, but you might also want to check the previous year's conference website just in case the detail wasn't properly copied to the new year website. In such a case you should still email someone on the organizing committee to verify the information is correct and inform them of the oversight.
As an example of a policy that is typical to my field, at ACM CHI full papers have a 10 page limit, and this does not count references (have as many as you like, but appendixes are not included in the main file at all). However, "work in progress" or "late breaking work" at CHI has a 6 page limit that does count references in the page limit, and this is typical for any of the extended abstract-style of submissions to this conference. It has been this way for at least a few years, though conferences evolve and often change their limits and what is included in the limit over time - so you have to check to be sure nothing changed every single year.
In short: there is no single ACM standard, and you should not assume anything and instead verify with current official conference materials what the rules are (many conferences will automatically reject your submission if it violates these basic rules, though not all). Verify with someone officially involved with the conference if the material is not already listed clearly on the conference website in call for participation/papers or the author guide.