I worked for a time doing "transitional project management" read "once we acquire you, figure out what we are keeping and jettison the rest". For a company that wasn't the size of Microsoft but grew through acquisition to be quite large.

In that particular case, which may or may not have any similarity to this incident, layoffs were determined in 2 ways

A) if you had 2 redundant or similar operational centers you got rid of the most expensive one unless there was some compelling reason not to (huge client demanded they stay, or they were more expensive but dramatically more efficient)

B) technology centers the goal was to integrate what had to be integrated as fast as possible. 80% of the time this meant literally telling people they wouldn't have the same functionality after the acquisition and getting rid of the entire support team. Occasionally, you would find some piece of technology that was great and easily integrated into the rest of the systems and that support team would be given good terms to stay until that was done. Very rarely you would find some piece of technology that was newly vital and hard to integrate, in that case you'd keep a skeleton crew to keep it operational, but you knew that it wasn't going to get any new resources and if it ever become non-vital that team was gone as well.

For point A) above, I've literally seen the decision come down to the cost per square foot of the office space. I've never been in a situation where they had a predetermined number they had to get to though. The number was a natural calculation based on the above.