$storage_definition = $node->getFieldDefinition('body')->getFieldStorageDefinition();
$storage_definition->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED);
$storage_definition->save();
$storage_definition = $node->getFieldDefinition('body')->getFieldStorageDefinition();
$storage_definition->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED);
$storage_definition->save();