Hello @hhnursery ,
Shipping rates based on quantity are not possible with Shopify flat-rate shipping.
However, it is possible to configure rates based on weight ranges or price ranges.
You can configure the weight for each product and set up shipping costs based on weight ranges.
For example, configure the product weight as 5kg and set the shipping rates as follows:
0-5kg - $10
5-10kg - $20
etc.