This is a tough question to answer because processes can have 100s of operators or 1 operator and this can affect the resource usage. My testing shows a waiting state operator uses an insignificant amount of memory and cpu. That being said, keep in mind that a process in a running state consumes a lot more space at the database level than a completed process. But all of this in mind, I think you can have a large number of waiting operators before you are going to see major performance degradation.