How to change the number of categories from the admin panel
Edit the file called “wp-admin/categories.php”
Look for the code below (should start around line 139)
$pagenum = absint( $_GET[’pagenum’] );
if ( empty($pagenum) )
$pagenum = 1;
if( !$catsperpage || $catsperpage < 0 )
$catsperpage = 20;
Change $catsperpage from the last row to whatever you wish.