15 years ago we placed the largest server order in our history: 1680 servers, packed into the now infamous "corkboard" racks that packed four small motherboards onto a single tray. (You can see some preserved racks at Google in Building 43, at the Computer History Museum in Mountain View, and at the National Museum of American History in DC, http://americanhistory.si.edu/press/fact-sheets/google-corkboard-server-1999
At the time of the order, we had a grand total of 112 servers so 1680 was a huge step. But by the summer, these racks were running search for millions of users. In retrospect the design of the racks wasn't optimized for reliability and serviceability, but given that we only had two weeks to design them, and not much money to spend, things worked out fine.
BTW the four white pins in the forefront are the reset buttons for the four servers, and the disks are mounted on a plexiglass board which lies on top of the board (not attached -- there was no time to design a proper bracket).