Get the excerpt of post_content

I’m sure if you did some more research you would find this as it’s been asked many times before.

If you just want all expert to return the value of 15 then you have to add something like this to your functions.php

// Customize excerpt word count length
function custom_excerpt_length() {
    return 15;
}
add_filter('excerpt_length', 'custom_excerpt_length');

This will make all excerpt return 15 characters.

Updated:

I don’t see the_excerpt in your code so you will have to add this aswell

<?php the_excerpt(); ?>