There is simple way to add label of Quantity in front of add to cart button.,

Add this code into functions.php.,

add_action( 'woocommerce_before_add_to_cart_button', 'echo_qty_front_add_cart' );
 
function echo_qty_front_add_cart() {
 echo '<div class="qty">Quantity: </div>'; 
}

and add this too at the bottom of the style.css.,

div.qty {
    float: left;
    padding: 10px;
}

Finally the result is.,