Set custom default qty when creating new product Magento 2.3

You can create your own modifier pool in your custom extension to achieve this. Following files would do the work.

etc/adminhtml/di.xml

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <virtualType name="MagentoCatalogUiDataProviderProductFormModifierPool">
        <arguments>
            <argument name="modifiers" xsi:type="array">
                <item name="defaultQtyModifier" xsi:type="array">
                    <item name="class" xsi:type="string">VendorModuleUiDataProviderProductFormModifierDefaultQtyModifier</item>
                    <item name="sortOrder" xsi:type="number">200</item>
                </item>
            </argument>
        </arguments>
    </virtualType>
</config>

Ui/DataProvider/Product/Form/Modifier/DefaultQtyModifier.php

<?php

namespace VendorModuleUiDataProviderProductFormModifier;

use MagentoCatalogUiDataProviderProductFormModifierAbstractModifier;
use MagentoCatalogModelLocatorLocatorInterface;

class DefaultQtyModifier extends AbstractModifier
{
    public function __construct(
        LocatorInterface $locator
    ) {
        $this->locator = $locator;
    }

    public function modifyData(array $data)
    {
        $model = $this->locator->getProduct();
        $modelId = $model->getId();

        if (!isset($data[$modelId][self::DATA_SOURCE_DEFAULT]['quantity_and_stock_status']['qty'])) {
            $data[$modelId][self::DATA_SOURCE_DEFAULT]['quantity_and_stock_status']['qty'] = 12;
        }

        return $data;
    }

    public function modifyMeta(array $meta)
    {
        return $meta;
    }
}

Here, instead of 12, you can defined your custom qty number.

Replace Vendor and Module with your original Vendor and Module name.

Hope this helps.