This fully documented solution shows how to use a process controller Orchestration to limit the number of running Orchestrations inside BizTalk Server 2004. This is similar to Application Pools in BizTalk 2002 that are no longer available in 2004. In addition, the rules engine is used to allow for real time updates to the number of running instances. This was an entry in the BizTalk 2004 Design Contest that ended in September 2004.
This sample shows a good use of the Business Rules Engine, Convoys, and Orchestrations.
This should work for both BizTalk 2004 and BizTalk 2006.
Get more information from the original blog post on this topic: https://www.biztalkgurus.com/biztalk_server/biztalk_blogs/b/biztalk/archive/2004/09/01/how-to-limit-the-number-of-running-orchestration-instances-in-biztalk.aspx