Stuart,
This depends on your settings. In the Workflow Preferences, under the Messenger group, you will find an option to control the number of instances to run. By default, the maximum number of instances is the same as the number of cores in your system, but you can change this if you need to.
Note that it's not recommended to have too many instances running at once. Not only will they share cores and not run any faster, but it will actually make processing exponentially slower the more instances run at the same time (unless this has changed since version 7.1).