Particle accelerators: how many do we really need?

There was some good debate in the comments of this post on the newly finalized plans to build the world's largest particle accelerator, the International Linear Collider, which would provide more precise measurements of certain collisions than the Large Hadron Collider. The LHC, as you may recall, found the Higgs Boson in 2012.

One point that seemed worth its own thread: how many particle accelerators do you think we need to build? How many should be built? These things aren't cheap, but compared to how much world governments spend on other things, the investment is small, not to mention typically divided among several countries.