Add the following snippet to your functions.php file in your theme.

    add_filter ( 'woocommerce_before_cart' , 'allow_single_quantity_in_cart' );
    function allow_single_quantity_in_cart() {
  
            global $woocommerce;
            $cart_contents  =  $woocommerce->cart->get_cart();
            $keys           =  array_keys ( $cart_contents );

            foreach ( $keys as $key ) {
                    $woocommerce->cart->set_quantity ( $key, 1, true );
            }
          
    }