Afaik 2 machines are almost mandatory
1. Dev machine
2. Email machine
The 2 biggest orgs inside MS are Office and Windows, devs in both of them can't have a single machine doing both the above tasks. (And VMs suck for disk based I/O, and that becomes a factor when you have multi-hour build time)
But, not everyone at MS works with windows, which is why I was a little surprised about the statement that everyone had more than one machine.