The EngineThreads property is a property of each Data Flow task. This property defines how many threads the data flow engine can create and run in parallel. The EngineThreads property applies equally to both the source threads that the data flow engine creates for sources and the worker threads that the engine creates for transformations and destinations. Therefore, setting EngineThreads to 10 means that the engine can create up to ten source threads and up to ten worker threads.
To understand how this property works, consider the sample package with three Data Flow tasks. Each of Data Flow task contains ten source-to-destination execution trees. If you set EngineThreads to 10 on each Data Flow task, all 30 execution trees can potentially run simultaneously.