cloud-foundry java spring spring-boot

Spring Boot – Limit on number of connections created

I developed a microservice using Spring Boot. I was performance testing the service by stubbing the backend calls. When I looked at the thread count , I see that the maximum number of threads that created to the service is 20 at any point in time even though the number of calls made is much higher. Are there any limitations with respect to number of calls that can be made to a microservice developed using Spring Boot. Please can you guide in what steps I need to follow to troubleshoot / increase the number of connections accepted by the service?