I was recently surprised to learn that they make server systems with up to eight PCIe x16 slots.

We were looking at this particular beastie [1] to host some Nvidia Tesla K40s for some simulation software. It would be a very expensive box, but the sim software costs a lot more.

[1] http://www.supermicro.com/products/system/4U/4027/SYS-4027GR...