Mambo 4.5.0 ONLY
This FAQ will show you how to change the default value for content item display. The default value is 10.
In the components/com_content directory you'll find the content.php file. On about line # 132 is this line:
$limit = trim( mosGetParam( $_REQUEST, 'limit', 10 ) );
change it to match the line below:
$limit = trim( mosGetParam( $_REQUEST, 'limit', 20 ) );
$limit = trim( mosGetParam( $_REQUEST, 'limit', 20 ) );
...or change 20 in the line above to whatever you like.
This will adjust the default value of 10 items to whatever you set as the value in the line above. If you did a copy and paste of the line above, then your new default value will be 20 items.