I know the CICD code I wrote and manage at work launches approximately a million docker containers a day, so I suspect the total number of docker containers used per day is well into the billions.
We have a suite of about two thousand integration tests and we run each test in its own docker container for isolation purposes. Multiply by 500 jobs a day and you get to a million containers.
Or many builds of the same app. Think compatibility matrix testing: take one suite of tests and run them against every one of hundreds of permutations of versions of their dependencies, one container each.
Not GP, just hypothesizing. This is what Travis matrixes do, for example.