How Many onsite candidates

How many candidates does Microsoft calls for an on-site interview? Any hiring manager here who can clarify? Each onsite candidate costs $$$ to the company, so im sure there would be a criteria. In my company, we call only the top 10% of the applicants. It would be great if a hiring manager can shed some light. In my case, i had 2 tech, 1 lunch with GPM, and 1 unscheduled post lunch interview with HM. In the end, the HM told me that in worst case, I 'may' have to undergo another (5th) interview on skype or else they will let me know the result in 2 weeks. Not sure why they need 2 weeks for results.