How do I delete 400 blank pages at end of my document

If this is a Page Layout document, display the page thumbnails, (View>Page Thumbnails). Click on the thumbnail of the first page to be deleted, scroll down to the bottom of the list, Shift-Click on the last one and press the Delete key on your keyboard.

If this is a Word Processing document, then as VikingOSX says, there's no 'convenience feature'.

Create a new document, and copy/paste all pages in the existing document up to those 400 blank pages. Pages v5 - v11.2 has no convenience feature to remove even one page and 400 will disappoint you immensely.