There is actually a place in App Architect where there are some configurations available. In the Portal Manager, when you are editing the page, click on the "Configure" button in the "Template Information" area. You can set "DefaultVisibleTabCount" to set the number of tabs that show up outside of the More Tabs tab - 6 is the default. You can also set settings such as "IncludeMiddlePane", "IncludeMoreTabsTab" etc. to further control the tab area.