start many instances at the same time,it seems that activiti cannot queue the request,it will start the next instance until the before instance's execution ended.
Async continuations (which are to be implemented in the near future) can help here: putting the first ctivity asycn would cause the thread to immediately return and hence the process starts are put in a queue.
But a simple ThreadExecutorPool in Java could do exactly the same as a current workaround.