Systems can be complicated and even the smallest detail can be dangerously revealing to a scrutinizing eye.
"Allied intelligence noticed each captured tank had a unique serial number. With careful observation, the Allies were able to determine the serial numbers had a pattern denoting the order of tank production. Using this data, the Allies created a mathematical model to determine the rate of German tank production. They used it to estimate that the Germans produced 255 tanks per month between the summer of 1940 and the fall of 1942."
"Allied intelligence noticed each captured tank had a unique serial number. With careful observation, the Allies were able to determine the serial numbers had a pattern denoting the order of tank production. Using this data, the Allies created a mathematical model to determine the rate of German tank production. They used it to estimate that the Germans produced 255 tanks per month between the summer of 1940 and the fall of 1942."
One source of many: https://www.wired.com/2010/10/how-the-allies-used-math-again...
This information was used to estimate force size and thus counter it, and it turns out this method was surprisingly accurate.