How do I add numbers onto an image, like in a car manual to identfy different parts i.e. 1, 2, 3 ...
Simply add text layers.
You need to activate the horizontal or vertical type tool, select the font, and the pt size.
Depending on your project, you may wish to add a description (text & numbers) to identify an object. You can add additional space which will not encroach on the object by enlarging the canvas, then adding the text in the extension.
Go Image>Resize>Canvas size. The dialog allows one to scale the extension in %. I usually start with 25%. Press the anchor point in the dialog to extend at the top or bottom. Of course, you can extend side to side as well.