At risk of being overly general....
Not all objects are collections, but all collections are objects.
- Ok, someone will mention a language where the words are defined differently.
But we're talking about names that make sense to people. So, um, hell yeah, if people will see it as a collection of like things, give it a plural name. Or give it a descriptive collection name like KeySet or BookList.
The convention of naming collections with a singular name is idiomatic and popular for people who design SQL Schemas. In that environment code does not so often have to make the distinction between the set (collection) and the individual member item. You strive to always do set operations when possible.
If your language/community has a tradition that the cool kids use singular for sets, as SQL does, then that's a nice choice.
In an OO language you often have to make the distinction between Cars and Car for example. So it's far more common to see collections named as plural.
But the overriding answer for this is, hey, it's really a matter of taste. The biggest error is worrying too much about what we all think about your names. Make the call, live with it a while, see if you want to do it the same way next time.
Most experienced developers will ignore your specific taste most of the time and care more about whether your style is reasonably consistent so that a reader can learn your code without constantly wondering whether you mean the same thing each time.
You are overthinking this. Decide and move on.