I worked for some time for one of the largest companies in the world during its biggest growth period. We had about 40,000 employees when I joined and about 15 years later we had about 120k. Honestly, we didn't really do anything significantly different production wise at the end that we weren't doing that the beginning, perhaps 10k of those extra 80k employees contributed to an actual increase in delivered products and services.
At the beginning a department was generally a manager who reported to a VP or GM, they had 5-6 managers under them and each manager had 6-12 employees. Those first line managers were responsible for making decisions and accountable for the results. About half the company was in manufacturing or customer support of some kind.
What we had at the end were lots and lots of meetings with lots and lots of people who all wanted a vote. Ownership and accountability were all over the place. Perhaps 3-5 people were all doing the job that one FLM was doing at the beginning. We had a ton of process and paperwork. Lots and lots of middle management. There were now as many as 7-8 layers between a first line manager and a GM or VP. That's another thing. I think we had about 9 or 10 VP's at the start and we had about 50 of them towards the end. We spent millions, even billions on things we really had no core competency on and then abandoned them when the people running them realized the quagmire they were in was about to go over their heads. We got further and further away from profitable products and services.
Then we took a seriously wrong turn innovation-wise (like we didn't do any for a while, just insisted on doing the same stuff we'd always done, the way we'd always done it) and we almost had our lunch eaten by a far less capable competitor. We lost or laid off about 30,000 employees in just a few years. Unfortunately many of them were the talented people who just didn't need to deal with uncertainty or bureaucracy anymore. Miraculously, a small group of employees coughed up a major innovation and we got back into the game and came back gangbusters. The company has such a commanding lead in the market they're in and are so efficient at manufacturing that really nobody else can profit in the segment so they'll maintain inertia for at least another 3-5 years, maybe more.
The company is still doing well, but frankly even at current employee levels you could take another 20-30k of the middle management and redundant "stakeholders" out to the parking lot, tar and feather them and not allow them back into the building ever again and absolutely nothing bad would happen. As long as you held onto the manufacturing, IT, customer service, engineering and about 50 marketing/PR people, things would go at least as well.
We worked with Microsoft a lot and I met regularly with their execs and senior management. They have pretty much the same disease. A long in the tooth cash cow that turns out money like a broken ATM and management that's sure all of that is due to their guidance and genius. Extreme narcissism and an ivory tower that goes to the moon. Most of the key decision makers and innovators are probably mired down in 7.5 hours of meetings a day and spend the other hour and a half doing e-mail and writing progress reports. Once they wander too far away from the cash cow, they burn through money and get nowhere. They absolutely fit the saying "when you're a hammer, everything looks like a nail". Its all about "how do we stuff Windows into it, and lets just try to do the same things that others already squeezed the profit out of, whether there is a real strategy there or if it even fits into anything we have any competence in".
So I'd say that with their current reasonable and profitable product set, they probably need around 65k-70k employees. I don't think at this point that they really have any valid position in the hardware business. The mobile market blew past them 3+ years ago. They might make 4th or 5th in the ecosystem business if they tried hard. They could easily be pushed right out of business in under 5 years.