Number of executable tests at same time are as same as registered number of docker nodes. When exceeds the limit, rest of the test will be delayed. Measuring the load is important, because long task will easily block the other many tests.
$ sqlite3 /var/lib/drone/drone.sqlite "select count(*) from jobs where job_status = 'pending';" 2
if you want to get more detailed information,
select repo_owner, repo_name, repo_private, build_branch from jobs inner join builds on jobs.job_build_id = builds.build_id inner join repos on builds.build_repo_id = repos.repo_id where job_status = 'pending'
I created the bot that is watching the pending tests, and notify to the hipchat.