limit how many words show up in the_content on index

Try wp_trim_words() https://codex.wordpress.org/Function_Reference/wp_trim_words

You won’t be able to use it with the_content() though because it echoes the content. You’ll want to use it with get_the_content() which just returns the info.

So it’d look something like this

echo wp_trim_words( get_the_content(), $num_words, $more_text );

Edit:

It’s good to note that the_content and get_the_content will return/echo any HTML that is in your content, while the_excerpt or get_the_excerpt will return only the text. So use the one that best fits your needs on how many words you want to return and whether or not you care to include HTML (such as images or embeded videos) in your output.