At Compose, your bill is determined by your exact number of monthly tested users (MTUs).
We define an MTU as anyone who enters your experiment triggering the test (whether they receive the control or a variant). It's important to note that this number will not necessarily be the same as your site visitors, because we are only tracking visitors who trigger an experiment.
For example, if you are running an experiment on the product page (PDP), then users who land on the homepage, but bounce before the PDP are not tracked toward your monthly total. Or if you specify an audience for your experiment (i.e. mobile devices), only visitors on mobile devices will be tracked toward your monthly total.
All visit events are tracked for all of your experiments within all projects of your organization. These are totaled up and visible on the billing page (for owners and admins). You can also see a breakdown of usage by project if applicable.