Under Content Source, you select Content Categories.
Under Content Mode Settings, you enter the category ID number (or numbers, comma separated) of the category (categories) you want to include. You can see the category ID numbers in Joomla (Content > Categories). You can enter more than 30 categories there, I tested it. Just make sure they are comma-separated.
Then in "How Many Articles to get from Section/Category" you select how many articles to get from the category (categories).
There's no setting that allows you, for example, to get 5 articles from category A and 10 articles from category B.
If the window becomes filled with loads of the articles in a vertical column, maybe you have HTML errors in some of the articles? This can break the entire website layout. Check this by temporarily unpublishing the articles and checking the output.
Regards,
Milos
