Shipping Rule Not Working For Rate API

Shipping rate API always returning "USPS priority" value when quantity is 1 or 2.
As per shipping rule that should return 3 values UPS ground, USPS priority, USPS first class when quantity is 1.
When quantity is more than 2 then it's fails to calculate shipping option.
What should change in API request JSON or any configuration for proper shipping Info base on shipping rule?