Little’s law states that the long-term average number L of customers in a stationary system is equal to the long-term average effective arrival rate λ multiplied by the average time W that a customer spends in the system.
L = Average number of customers in a stationary system
λ = Average arrival rate in the system
W = Average time a customer spend in the system
In context of an API it means:
L = Average number of concurrent requests system can serve
λ = Average arrival rate of requests in the system
W = Average latency of each request