add_filter('woocommerce_available_payment_gateways', 'unsetting_payment_gateway', 10, 1);
function unsetting_payment_gateway( $available_gateways ) {

    // HERE Define the limit of quantity item
    $qty_limit = 1;
    $limit_reached = false;

    // Iterating through each items in cart

        if ( WC()->cart->get_cart_contents_count() > $qty_limit ){
            $limit_reached = true;
        }

    if($limit_reached){
        // HERE set the slug of your payment method
        unset($available_gateways['cod']);
    }
    return $available_gateways;
}