For DevOps questions, you'll get the best answers from the DevOps forums where the Azure DevOps team and DevOps community are actively answering questions:
https://developercommunity.visualstudio.com/spaces/21/index.html
Having said that, I believe that this may be the documentation that you are looking for:
Microsoft-hosted agents > I need more agents. What can I do?
https://learn.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops&tabs=yaml#i-need-more-agents-what-can-i-do
"All Azure DevOps organizations are provided with several free parallel jobs for open-source projects, and one free parallel job and limited minutes each month for private projects. If you need additional minutes or parallel jobs for your open-source project, contact support. If you need additional minutes or parallel jobs for your private project, then you can buy more."