I don't know of any straightforward way to do this via the API. But if you get an account on Tool Labs,[1] it's a simple SQL query: SELECT MIN(page_id), MAX(page_id) FROM page.

 [1]: https://tools.wmflabs.org/

 

If a page is deleted and then undeleted, the page id might change.

 

If the page is deleted, yes. Special:MergeHistory will probably eliminate one of the involved page ids too.