There’s no such function of Paging in Document Library.
Even if I defined the count of items on modern UI, it can just display more by scrolling the Scrollbar instead of using the paging button.
It would be really helpful if the paging button can be added like on classic UI.
And also, when I group items by using collapsed in Document Library, it only can display one group on modern UI.
It would be really helpful if all can be displayed like classic UI.