Earlier quoted context omitted.
But if you design a factory to make shells quickly, each production line can probably do 1 shell per second (they're not so different from big bullets, and those are made very rapidly). 11 of those lines and you can be keeping up that rate forever.
It's not enough to produce them, you also need to get them close to the front. The logistics of transporting millions of tons ammunition outside of normal trading routes should not be underestimated
Just happened to be listening to Dan Carlin's hardcore history about WWI. From memory, the Germans would build multiple railway lines to the front to supply a large offensive like this. And then they'd spend weeks stockpiling ammunition and food using those multiple railway lines. At times this conferred a large advantage to the Germans as parts of the Allies side would be supplied by a single road vs. multiple railway lines.