class Allow_Only_1_Product_Cart_MYK {
    function __construct() {
     add_action( 'woocommerce_before_calculate_totals', array($this,'wcaopc_keep_only_last_cart_item'), 30, 1 );
    }
  function wcaopc_keep_only_last_cart_item( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;
    if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 )
        return;
    $cart_items = $cart->get_cart();
    if( count($cart_items) > 1 ){
        $cart_item_keys = array_keys( $cart_items );
        $cart->remove_cart_item( reset($cart_item_keys) );
     }
   }
}
$plugin = new Allow_Only_1_Product_Cart_MYK();