Concurrency not working

We’re trying to configure concurrency for our CI pipelines in order to run multiple jobs in the same instance, but Docker Machine launches instances that only run single jobs. Any clues?

Here’s our abbreviated config:

concurrent = 50

[session_server]
  session_timeout = 1800

[[runners]]
  limit = 0
  request_concurrency = 5
  executor = "docker+machine"

  [runners.docker]
    [runners.docker.tmpfs]
    [runners.docker.services_tmpfs]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
  [runners.machine]
    IdleCount = 1
    IdleTime = 1800
    MaxBuilds = 10