I know of no "repeat last command" keyboard shortcut.
If you are trying to avoid going into the menu system (because it can be tedious), you can assign keyboard shortcuts to any items in the menu that don't already have them. You can also change the ones that are assigned by default by Numbers. You do this in System Preferences, not within Numbers.
Before doing that, though, there are a lot of keyboard shortcuts available that are not shown in the menu system. Inserting a new row below the currently selected cell/row is Option Down Arrow. Look in the Help menu under Keyboard Shortcuts for a long list of shortcuts.