Our department has staff of 300. We maintain 150 mainframes (z/OS) and handful of z/TPFs and a handful of z/VM servers supporting several thousand z/Linux guests. We have 200,000 internal users and millions of customers via ATMs, POS networks and other interfaces. If we never had changes, the systems would run for years without maintenance (except for storage additions). Thankfully, we have about 1000 changes per week, so I am still employed.