Why does the number on the app icon change?

That's the number of your open items across all active lists. It changes when you or other participants add, check off or clean up items.

You can turn it off in the app settings (profile icon at the top right of the overview) under „Items total on app-icon“.

You can keep a single list (e.g. a large pantry or archive list) out of the count specifically: in that list, via „⋯“ at the top right → „List settings“ → „Customization“ → „Exclude items“.