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.
When you don't have to deal with boring things like permission to build, consent of landowners, keeping roads open, etc., building a railway is very quick and easy.
Building multiple railway lines is also a safety thing - you don't want to leave a whole section of your troops unsupplied just because some train is hit by a shell and wrecked blocking the tracks.
Stockpiling those shells for a "huge burst" at the start of a battle is more than possible though.
The main issue today, is such stockpiles are easily found by recon (satellites, airplanes, drones), and then blown up by enemy artillery / rocket artillery.
11 of those lines and you can be keeping up that rate forever.