Supporting multiple instances of one activity can enable workflow management system to be flexible on handling workflow process. When handling multiple instances the main problem is the schedule of instances. After analyzing the assignment and the join of multiple instances, this paper proposes the context of workflow activities and then formally describes the semantic of a multi-instance activity. Base on the formal presentation, the concept of Shell is proposed to control the assignment and submission of the multiple instances. The Shell can control the multiple instances to run synchronically and control the progress of workflow process according to the semantics of activities. The Shell gives a solution to schedule and control of activity multi-instances in workflow process.