The maximum number of automated Selenium and Javascript unit tests (API initiated tests) that can be run concurrently is limited according to your subscription plan. See Pricing Table and Plans
We understand the importance of running as many tests concurrently as possible to increase the velocity of your continuous integration flow. To support this, we do not place arbitrarily low constraints on the number of parallel tests. A major advantage of our service is that we provide a combination of real operating systems and an extensive array of physical phones and tablets. While we do maintain a large number of mobile devices, they are not unlimited. Ensuring the load is spread across the configurations is important in providing availability for all our customers.
We provide a high number for maximum parallel sessions and these must be spread across separate configurations. Spreading your tests across multiple operating systems and devices ensures the highest concurrency possible under your plan.