What is the ratio of Commercial + ATP pilots with employment as pilots in the US?
You can find the full database of FAA information at this link but more useful they offer some statistics here.

As of 2018 there are 162,145 individuals that carry an ATP level ticket and 99,880 Commercial tickets out there. 

The Bureau of Labor Statistics provides some employment stats however their stuff only goes up to 2016. As of then they claim there are 124,800 jobs for the title "Airline and Commercial Pilots" If we look back at the FAA's 2016 data we will see that in 2016 there were 157,894 ATP tickets out there and 96,081 Commercial level tickets. Assuming that BLS rolls up quite a bit under "Airline and Commercial Pilots" including 135 ops, flight instructor and full on 121 jobs there are 124,800 jobs for 253,975 possibly qualified people. 

So there are roughly 129,175 people with ATP or Commercial level tickets not working in ATP/Commercial Aviation jobs. 

Note: This does not account for people over 65 (data that it does not appear is released) as well as people who may have lost their medical or otherwise cant fly. However the FAA uses the phrase "ESTIMATED ACTIVE"  which could mean they account for all the above noted things.