Here is five minutes of the MIT Build Servers today as of a few minutes ago (11:52 am CDST). Usage is heavy but not severe at this moment. Occasionally, I have seen average usage for short periods of time around 7.
When the notice Please try again in a few minutes appears, things are very busy; try again. Notice there are peaks and troughs to activity. If you attempt to build during a trough, you are more than likely to succeed; unfortunately no one can predict when a trough will appear.
The maximum builds possible on each server is 8 (red marked row); the number of builds simultaneously taking place on individual servers (green marked row) in the bank of 15 servers varies with the load//number of developers using App Inventor 2 to build their apps.
If you can't build; wait a while and you will be able to finish your app. You are sharing the build servers with lots of developers.
As Tim mentioned, MIT recently added 3 servers to mitigate issues. Evan recently indicated "Peak usage is typically around between 9am and 11am US Eastern time"