The author uses numbers so many times in this book. In some cases, he could be trying to show how everything in oskars life is numbered( Number of Blacks in New York..etc).
There are some pages in the book where there are just straight numbers and not passages or letters.
"I've hammered a nail into that bed every morning since she died! It's the first thing I do after waking! Eight thousand six hundred twenty-nine nails!" (pg 161) this is a quote by A.R. Black, the old guy with the eye patch.
When the grandfather of Oskar is calling his wife after the death of their son. With his inability through numbers on the pay phone he communicates with her to create the sentences he wants to convey to her. "I broke my life into letters , for love I pressed '3, 3, 2, 8, 4,' when the suffering is subtracted from the joy, what remains? What, i wondered, is the sum of my life."
With numbers it is also a way of collecting things and keeping track of what is most important to him. Which is a coping mechanism to both Oskar and his grandfather.
Numbers are a way that the characters try to connect with other people when they cannot use words. Oskar thinks of numbers to feel closer to his father and his grandfather tries to connect with his grandmother. Both characters are having trouble expressing what they truly feel and resort to numbers instead.