This is probably something IOS development newbies might encounter while designing an IOS app graphical interface. By default, when adding a Toolbar to the main storyboard, a single bar button is included as shown below.
I wanted to add more bar button items to the toolbar; and I look high and low for a way in the property panes on the right of Xcode. Eventually, I figured that the bar button items need to be dragged from the Object library and dropped onto the toolbar on the main storyboard, as shown in the screenshot below.
The screenshot below shows three bar button items on the toolbar.