Use these options in /etc/gitlab-runner/config.toml:
# Limits how many jobs can run concurrently, across all registered runners.
# Each [[runners]] section can define its own limit, but this value sets a maximum for all of those values combined.
# For example, a value of 10 means no more than 10 jobs can run concurrently. 0 is forbidden.
# If you use this value, the runner process exits with a critical error.
concurrent = 6
[[runners]]
# limit option can limit how many jobs can be handled concurrently by this registered runner.
# 0 (default) means do not limit.
limit = 3
# Limit number of concurrent requests for new jobs from GitLab. Default is 1.
request_concurrency = 3