Java async thread pool size

Hi All,
Anybody any advice for working out the max queue size for a java thread pool using async. I have a request coming in 2 per second and a max response time of 11ms and min response time of 2ms. There are 10 threads and queue size of 2000 and it handled it fine during a 1 hour soak test but want to refine the numbers if possible. I initially had a thread count of 5 and queue size of 500 but this resulted in a large number of 503 errors from the microservice.
Thanks