A few weeks ago I had the pleasure of being part of a team that was migrating 50+ active projects from Team Foundation Server 2008 as well literally hundreds of build definitions. Beyond the build definitions they had created a number of custom tasks to extend some of the build capabilities for complex deployment to multiple environments, custom metrics, you name it.
They did it with one TFSBuild.proj file. Make no mistake. One. Impressive work Tim Stall.
The challenge of hundreds of build definitions
The company had structured their build definitions by product and then by branch, then branch environment (per branch). Perhaps this picture will better explain it.
As you can see, it's an ugly mess and just begging for organization. What's a programmers best organization tool? Tree views.