In the sentence:
Many of us are even unable to remember our own phone number
Assuming each person only has one phone number, which is grammatically correct: phone number or phone numbers?
In the sentence:
Many of us are even unable to remember our own phone number
Assuming each person only has one phone number, which is grammatically correct: phone number or phone numbers?
Good question. You need to understand that the confusion created is because you used the word 'own'. In this situation you must say "Many of us are even unable to remember our own phone number." (in singular) Of course if you use plural it won't be grammatically wrong, but semantically it won't be correct. (If you're supposing that every one has one number.)
If you haven't used 'own', you must have used the plural form. (unless the object can be referred to as one common object, like saying: "We live our lives." and "We live our life." In second sentence we used the singular because what's meant by life is the human being life.
Hope I provided you with what you need.
"Number" could be either singular or plural, both logically and grammatically. Consider the case of a dorm where there is only one phone for many people. The interesting case is where each person has a different number and there is only one phone per person -- then, can you use a grammatical singular? I think you can, as a matter of fact. I know of no relevant grammatical principle.
"Phone numbers."
Let's take "own" out and make it more obvious.
Many of us are unable to remember our phone number.
This suggests that we collectively have one phone number that many of us can't remember.
Many of us are unable to remember our phone numbers.
This suggests that we have multiple phone numbers -- which is true, and is what you meant.
Using singular for plural can be disorienting. "We all like to drive our car" -- we only have one car? "After all that noise, our head was in serious pain" -- how many people have that one head, anyway? Grammatically correct if there's only one car and one head -- but not in the case you meant!