To do load testing with a reasonably high load, ATS would not be the tool of choice for now. In principle, you could do a load test by creating a large dataset and running it in parallel, but the total number of parralel users is limited by the number of parallel sessions your Selenium service supports.
For instance, a trial subscription at Browserstack supports up to five parallel sessions, but you can increase this number to some extend by choosing another subscription type. However, load testing typically uses far larger amounts of concurrent sessions than you could create in this way.
Load testing is present in APM2, so we advise to take a look at APM2 for load testing.
Another alternative is to use HP Loadrunner TruClient, this has also been used by a number of customers to loadtest their Mendix applications.