As per Copy a Space  functionality , there is a limitation of 2000 pages in a single page tree

• From one branch of pages in the original space tree, only 2000 pages can be copied at a time. If the branch contains more than 2000 pages, the rest of the pages won't be copied and the copying will stop.
  • This limitation doesn't apply to blog posts as well as to the case where no branch in the original space contains more than 2000 pages.

Suggestion to increase this limitation