Given an arbitrary positive integer number n, you could create n-tab tabContainer thanks to Drag and Drop
About custom layout for each newly created tab, it's another story
Unfortunately, it does not yet support to newly adding tab at run-time and I have just created a JIRA issue for it https://jira.jboss.org/browse/GTNPORTAL-1417 ( So you can watch )
But you are able to pre-configure your own n-tabs layout by adding configuration in the portal.war/WEB-INF/conf/uiconf/portal/webui/container/ContainerConfigOption.groovy file. You could see the 2 and 3 tabs configuration fragments there for reference.