Change the default WordPress excerpt Length

You can change the default WordPress excerpt by applying a code filter in your child theme, for example:

function my_custom_excerpt_length( $length ) {
    return 25;
}
add_filter( 'excerpt_length', 'my_custom_excerpt_length', 1000 );

For more information you can check WordPress website:
https://developer.wordpress.org/reference/hooks/excerpt_length/