AFAIK, when you scale out multiple instances, the webjobs will run on the instances in parallel in separate processes. But there are prerequisites of it,
- The webjob should be continous and not Manual/Scheduled.
- For this to happen correctly, you need to be running in Standard mode, and have the Always On setting enabled.
Note: If you use TimerTrigger in your webjob, it will not scale out. Refer to this article.
Behind the scenes, TimerTrigger uses the Singleton feature of the WebJobs SDK to ensure that only a single instance of your triggered function is running at any given time. When the JobHost starts up, for each of your TimerTrigger functions a blob lease (the Singleton Lock) is taken. This distrubuted lock ensures that only a single instance of your scheduled function is running at any time.
For more details about the issue, here are two similar posts for you to refer, 1 and 2.