By default the scheduler runs on all instances, and that's not different at all to any on-prem and AMS based environment. For clustered instances (like the authoring instances in AEM as a Cloud Service) you have the option to run it only on the master.
Schedulers are event handling mechanism which can run on the instance as per the cron job provided. They would run on the instance if the code is present in them - even though adobe creates some extra instances [which would be a backup or replica of existing instances]
To know more on the annotations of different event handling mechanisms please check my blog here.