I use to have TOCs in articles when this site ran DokuWiki. But by default, WP doesn’t provide this ability. After a few Googling, I found the Table of Contents Plus plugin.
I choose this one for two main reasons:
- it can automatically insert a TOC in WP since “X or more headings are present” ;
- it has various looks embedded to match your theme.
You can have a look at a rendered example here.
Hope it helps.