Auto insert More tags in all posts

If you want to control the string count, then you can do it by the following.

Put the below code in the active theme’s functions.php file:–

function custom_excerpt_length( $length ) {
    return 20; // You can change the number here as per your need.
}
add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );

By default, excerpt length is set to 55 words. To change excerpt
length to 20 words using excerpt_length filter, add the above code
to functions.php file in your theme:

For more details check the Codex