Hi Susan,

Assign the style to a level in a MultiLevel List. The number will be at the beginning of the line.

If you use automatic paragraph numbering or bullets read Shauna Kelly's directions on numbering and bullets. Start with How to create numbered headings or outline numbering in Word 2007 and Word 2010. For large documents you must follow these directions or you will lose your hair!
(Mac version: Outline Numbering in Word for macOS - Best Practices - Brandwares)

This may seem a bit convoluted at first, but it really is not. Just follow the steps. Shauna Kelly's instructions use the built-in heading styles, but you can use any existing paragraph styles including your custom styles. There are, however advantages to using the built-in heading styles when you create a Table of Contents. Here are some more advantages: Why Use Word's Built-In Heading Styles? by Shauna Kelly Note, you can modify these built-in styles to look exactly the way you want.

Videos on this
https://www.youtube.com/watch?v=He_ob8ydc9E
https://www.youtube.com/watch?v=qbyTcWo52G4
https://www.youtube.com/watch?v=GevZOS-nCuw
https://www.youtube.com/watch?v=niD6VXPvAyU

The basic idea is that the numbering is set using the Define MultiLevel List dialog with each numbering level being attached to an existing paragraph style. Once you have this set up, you should not use the buttons for numbering in the Ribbon but rather apply the appropriate style for that level.

You can save a document with this as a template for future documents if you want so you will not need to do this every time.