Limiting the number of active builder is documented and possible using Interlocks.
http://docs.buildbot.net/0.8.1/Interlocks.html
While this is working well, there is a slight disadvantage because the slave is assigned and busy waiting for the lock to be available.
There is a simple solution by using custom nextBuild function. The following example is equivalent to using a buildbot master lock:

def myNextBuild(bldr, requests):
  if len(bldr.building):
    return None
  return requests[0]