So I'm the guy who runs it at Google. :) That number as far as clients was something we stated quite a while ago. It's a much larger number now. You can certainly scale Puppet up more. You just won't be running the one server. What we do is have a dedicated Puppet CA server, and then geographic masters in each major location.

Actually you should look at a newer generation of config management than cfengine. Try Puppet: http://reductivelabs.com/products/puppet/