They have 4 office buildings and one cafeteria/commons building on that campus - space for around 1500 employees.
It's worth noting that Microsoft's bay area presence is already pretty fragmented, though. They have some more office space a couple exits down 101 in Sunnyvale (not walkable to the MV campus at all), a Skype office in Palo Alto, Yammer in SF, etc. They've also announced plans to renovate and expand the Mountain View campus: http://www.bizjournals.com/sanjose/news/2016/01/21/microsoft...
It's worth noting that Microsoft's bay area presence is already pretty fragmented, though. They have some more office space a couple exits down 101 in Sunnyvale (not walkable to the MV campus at all), a Skype office in Palo Alto, Yammer in SF, etc. They've also announced plans to renovate and expand the Mountain View campus: http://www.bizjournals.com/sanjose/news/2016/01/21/microsoft...