Limiting on attaching number of work items as a child
Accepted answer
As I already answered in https://jazz.net/forum/questions/275752/if-parent-work-item-is-in-completedone-state-no-any-child-should-create-or-attach-to-it this would be a pre-condition also called Advisor. If the built in do not provide what you want (and they don't), it is possible to create custom advisors. They are Java Server Extensions. To learn how to develop them, perform https://jazz.net/library/article/1000 completely. Then see https://rsjazz.wordpress.com/?s=Advisor&submit=Search for the differences between a follow up action and an Advisor.
If you are interested in the APIs look at https://rsjazz.wordpress.com/2015/09/30/learning-to-fly-getting-started-with-the-rtc-java-apis/ and if you are interested in other extensions check http://jazz-community.org/