- We should wash our face with clean water.
- We should wash our faces with clean water.
Which is more natural, 'face' or 'faces'?
Does the plural subject 'we' require a plural noun in the object?
If each individual [item] implied by a plural subject has only one of something (each of us has one face, in OP's example), we tend to extend the plurality of the subject (we) to the object (faces). Thus:
But if you look at some of the results there, I'm sure you'll find that the second (singular) set will have more contexts where we're all taking pride exactly in the same thing (as opposed to each of us being proud of our specific version of whatever it is).
So in OP's specific example, most people in most contexts would use plural faces. But if preceding text had focused on "the face" in a more "generic, archetypal" way (for example, A stranger's face is usually the first thing we look at when we meet them), it wouldn't be unreasonable for the writer to continue with the singular when recommending that everyone should wash their face properly.
There's one additional point to consider in contexts like OP's. Sometimes (especially when addressing children), we use we when we really mean you (singular or plural, we do it in both contexts). Thus:
(Teacher to lazy pupil)
3: We didn't do our homework, did we?
(Dentist examining new client's mouth)
4: We haven't been using our dental floss, have we?
That interpretation (we means you, and doesn't include the speaker) isn't always possible. But it certainly could apply in OP's context, so bearing in mind this would represent a very condescending style of address, that might be one more reason to use the plural in that exact context.
In this case, I'd say you need the plural "faces". As, presumably, we each have our own face, you are talking about many faces here.
But it isn't true that a plural subject requires a plural object. Many people could be acting on one thing. For example, "We should keep our neighborhood clean." There could be many people all living in one neighborhood. Plural subject, singular object. One person could be acting on many things. "I should wash my hands with clean water." One person but he has two hands. Singular subject, plural object.
From the position of descriptive grammar, the usage that is most common in your dialect is more 'natural'.
From a technical perspective, the confusion arises due to a lost reference for possession.
To clarify, think of a family who owns one car-
We should all wash our car.
A family that owns several cars-
We should all (every one of us) wash our cars.
A family where each member owns a car-
We should all (each one of us) wash our (own) car.
A family where each member owns several cars-
We should all wash our cars.
Confusion arises because the last form is a grammatically correct expression. At the same time, unlike a car, it is implicitly understood that each one of us has only one face.
Read the following three sentences to see if you can understand how the singular 'face' makes sense when possession is understood clearly.
We should all wash a face with clean water.
We should all wash our own face with clean water.
We should all wash our face with clean water.
Notice how 'own' specifies what is implicitly understood, each one of us owns just one face.