Greetings @Bellinghamster ,
Achieving a shipping rate using Shopify’s Flat Rate Shipping based on quantity is not feasible.
However, you can opt for a weight-based approach.
For instance:
0-2KG: $10
2-4KG: $20
To implement this, assign a weight of 2KG to each product. Consequently, when a customer purchases two products, the shipping cost will display as $20.