Gitlab limit parallel pipeline execution

In brief - use the resource_group directive.

Example:

stages:
  - build

jobA:
  resource_group: jobA
  stage: build
  script: 
    - echo HelloA

jobB: 
  stage: build
  script:
    - echo HelloB

There are some useful patterns for Resource Group.

resource_group: $CI_ENVIRONMENT_NAME … Limit per environment

resource_group: $CI_JOB_NAME … Limit per job

resource_group: $CI_COMMIT_REF_NAME:$CI_JOB_NAME … Limit per job per branch

resource_group: $CI_COMMIT_REF_NAME:$CI_ENVIRONMENT_NAME … Limit per environment per branch (e.g. review apps)

Source Gitlab issue 15536