You'll need a 3rd table. You'll have your Products table, Line Items table, and Orders table.
In a restaurant, you have your main "Products", like Steak, Grilled Chicken, etc.
Think of Line Items like Line items on a receipt. A table of people order 3 Steaks. That's your line item.
The Steak is your actual Product and it's where you can put the Quantity. You link it back to your Steak product and you also link it to your Order table (for the custom receipt)
So your Product will be linked to your Line Items. Line Items will be linked to your Orders.
Here's a base template that might help you see the layout - Ecommerce Airtable template
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Hannah - On2Air.com - Automated Backups for Airtable