Sometimes you need to control number of Instances of an Orchestration get created. although BizTalk has its own mechanism for throttling but because of some external resource limitations like database connections, Cloud Connections limitation it is recommended to handle throttling from within the BizTalk Orchestration using Singleton Pattern, Fixed number of Controlled instances (Multi-Ton). There is one very fine blog/article written by Richard Seroter on this. Following is the link to that

BizTalk Orchestration Throttling Pattern