By default, excerpt length is set to return 55 words but if you wanted to change the default number of words, just paste this code snippet into your theme’s functions.php

add_filter( 'excerpt_length', 'my_new_excerpt_length', 999 );

function my_new_excerpt_length( $length ) {
    return 40; // change this with your desired value
}

Just change the 40 with your desired value.