Please check the TCA configuration of your news category field.
Go to Admin Tools -> Configuration (in your backend), choose $TCA from select menu on top of the module.
Open the node tt_news/columns/category/config and check the value for maxitems. If it is only 1, the TYPO3 behaviour is correct and you need to find out where you get this setting from, maybe from a setting in typo3conf/extTables.php or from an extension extending tt_news. Please double-check and report back.