[Lizard]

You want to set the numbering on the whole level, not an individual item.
1) Choose "Show Styles View" from the View menu.
This will add a column of dotted boxes to your document.
2) Click a dotted box in the header row of the document. This represents the style for all items at that level. This will select the items which currently are at that level, but will also apply to all items put into that level in the future.
3) When you've selected the dotted box for the level you want to modify, then you can go set numbering or any other styling option you want.
4) When you're done, choose "Hide Styles View" in the View menu.