I know a way to add multiple serial numbers in QuickBooks Online, FerCW.

We can track the serial numbers in the program by adding them to the Description field. If you track your items as inventory, this is only available in the Plus version. 

Here's how:

1. Go to the Gear icon.
2. Select Products and services.
3. Choose a product you'll want to enter the serial numbers.
4. Click the Edit link. 
5. If you haven't added it, click New.
6. Add the necessary details.
7. Enter all the serial numbers on the Description field.
8. On the Preferred Supplier field, choose the vendor's name.
9. Click on Save and close.

Once done, you can create invoices to track the item you sell to your customers.

In case you'd like to buy more items from your vendors in the future, I encourage creating and sending purchase orders to them. This helps you easily track your purchases.

Please let me know if you need more help in tracking all the details of your items. I'm only a few clicks away. Keep safe always!