Jenkins API call (or Groovy builtin) that returns list of all currently running builds of jobs
Turns out you can hit the following URL to get an XML output of all currently running builds, including their build numbers. 

http://jenkinsURL/computer/api/xml?tree=computer[executors[currentExecutable[url]],oneOffExecutors[currentExecutable[url]]]&amp;xpath=//url&amp;wrapper=builds


I greped it for a count and used that for my calculations.