how can I use "fewer" and "more" with numbers?

I have never heard anyone use words "fewer" and "more" when talking about the fact that a certain number of items is greater than a certain number of other items by a constant.

For example if the number of pens that I have is greater than the number of pencils by 5 what is the proper way to say it without using the word "number". Is it "I have 5 more pens than pencils." And "I have 5 fewer pencils than pens." Both of these sentences sound extremely weird and I doubt they are correct, so, what is the proper way of saying these 2 sentences?

Both sentences are grammatically correct.

"I have 5 more pens than pencils."

"I have 5 fewer pencils than pens."

For the record, they sound fine to me as well as a native English speaker who regularly has to compare numbers and quantities.

Oxford dictionaries includes "fewer than 50 people" in an example of word usage.