Sauce Labs bills customers for every minute of time used by one of your test sessions.
When your tests are running in parallel, they are using multiple test sessions. This means we bill you for each minute used by each of those sessions.
For instance, say you have 6 tests, each of which takes two minutes. If you run tests singularly, they will be finished in 12 minutes, and you will have had 12 minutes of Sauce Labs session time: 1 session x 6 tests x 2 minutes = 12 minutes.
If you run two tests at once, your tests will be finished in 6 minutes. However, Sauce Labs will still have allocated you 12 minutes of session time: 2 sessions x 3 tests each x 2 minutes = 12 minutes.