How Many Employees Microsoft Really Need ?

Yesterday, word came down that Microsoft was starting to lay off some 18,000 workers. As of June 5th, Microsoft reported a total employee headcount of 127,005, so they're cutting about 15% of their jobs. That's actually a pretty huge percentage, even taking into account the redundancies created by the Nokia acquisition. Obviously, there's an upper limit to how much of your workforce you can let go at one time, so I'm willing to bet Microsoft's management thinks thousands more people aren't worth keeping around. How many employees does Microsoft realistically need? The company is famous for its huge teams that don't work together well, and excessive middle management. But they also have a huge number of software projects, and some of the projects, like Windows and Office, need big teams to develop. How would we go about estimating the total workforce Microsoft needs? (Other headcounts for reference: Apple: 80,000, Amazon: 124,600, IBM: 431,212, Red Hat: 5,000+, Facebook: 6,800, Google: 52,000, Intel: 104,900.)