The Spring '15 documentation seems to imply that processes only fire once. The reason I say this is there is no corresponding explanation for processes in the same vein as the explicit explanation (shown below) for how a workflow rule can cause triggers to fire again.
As with many things in Salesforce, I like to trust and verify. And I found that Process Builder processes actually can cause triggers and the process itself to execute up to five (5) more times. I couldn't find this documented anywhere, but that seems to be the de facto limit on process recursion.
The proof
To prove this to myself, I decided to stage a manual test based on the following constructs:
- A custom object named TriggerEvent__c exists as a child to the Account object
- An Account trigger exists to create a TriggerEvent__c record before update
- A process exists that launches a flow, which increments the Num Employees value by one if Num Employees is not blank
Under this setup, when I updated an account and entered 100 for Num Employees, I actually ended up with 106 for Num Employees and six new child TriggerEvent__c records.