Russia produces 2million 152mm and 122mm shells per year. Rheinmetall alone is at 700.000 155mm shells per year by 2025 alone. The difference is not that big, but many European countries are increasing stockpiles and there is a need to train their own militaries, which requires firing some of them.

However the number of shells matters much less then the enemies they destroy. NATO artillery is much much more precise. So they need one shell to hit a target, whereas Russia might need ten. The other big part is air power. NATO unlike Ukraine can destroy Russia air defence and then use a lot of bombs to create a break through in the line. This just can not be easiyl transfered to Ukraine, as training pilots and ground crews takes years.