Production numbers of vintage technology have always been a somewhat mysterious subject. How many 4004 processors did Intel actually make? I am not even sure Intel knows. Unlike modern car companies who can track production numbers down to the shift of the day it was made, computer companies of the 70’s and 80’s were rather fast and loose with record keeping.
Thankfully with some research, serial numbers and some math (the famous tank equation) Michael Steil of Pagetable.com came up with what appears to be a very good estimate of Commodore 64 production. 12.5 million units, somewhat less then other numbers that have been thrown around, but backed by research and supported by math. Read how he came to the conclusion here.