You can see this limit in this forum - the page selection boxes just go up to 100. I have ported a database from2.1 to 3.3 successfully except that the Recent Discussion page will not let me look beyond that 100 page limit although the database showed 200 odd pages under 2.1
You can add this to your conf/config.php $Configuration['Vanilla']['Discussions']['MaxPages'] = 300; Note that some limit is reasonable to not overload the DB if you have hundreds of pages.