change the config.toml in /etc/gitlab-runner and add following:

1. in global section: concurrent = 100
2. in runners section: limit = 10

The first (in global) set the maximum runners on this system. The second (in runners) you set the limit of this specific runner.

Complete example:

concurrent = 100
check_interval = 0
 
[session_server]
  session_timeout = 1800
 
[[runners]]
  name = "Sonarqube Runner"
  url = "https://gitsrv.schnuerle.com:3453/"
  token = "L29dsfvr-gAdfsf5EvFt"
  executor = "docker"
  [runners.custom_build_dir]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
    [runners.cache.azure]
  [runners.docker]
    limit=10
    tls_verify = false
    image = "sonarsource/sonar-scanner-cli:latest"
    privileged = false
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache"]
    shm_size = 0