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