Queues are the enemy of high-velocity flow. When we see them in our software, we know they will be a performance limiter. We should look at them in our processes the same way.
I've seen meeting rooms full of development managers with a chart of the year, trying to allocate which week each dev project will enter the QA environment. Any project that gets done too early just has to wait its turn in QA. Entry to QA becomes a queue of its own. And as with any queue, the more variability in the processing time, the more likely the queue is to back up and get delayed.
When faced with a situation like that, the team may look for the ``right number'' of QA environments to build. There is no right number. Any fixed number of environments just changes the queuing equation but keeps the queue. A much better answer is to change the rules of the game. Instead of having long-lived (in other words, broken and irreproducible) QA environments, focus on creating a machine for stamping out QA environments. Each project should be able to get its own disposable, destructible QA system, use it for the duration of a release, and discard it.