First install & activate the List More Custom Field Names plugin.
Then add the following code to your functions.php or your custom plugin file.
// List more custom field names
add_filter( 'c2c_list_more_custom_field_names', create_function( '$limit', 'return 1500;' ) );
You can change the 1500 in the example to any number you would like and that is how many custom field names will be displayed in the drop down when you add a new custom field to your post/page.