We have 4 servers: Dev, Stage, Test, Prod.
We are getting one free CI/CD according to Microsoft:
1 Microsoft-hosted job with 1,800 minutes per month for CI/CD and 1 self-hosted job with unlimited minutes per month
$40 per extra Microsoft-hosted CI/CD parallel job and $15 per extra self-hosted CI/CD parallel job with unlimited minutes
We are using a self-hosted agent. The question is how many CI/CD do we need to buy? We don't need parallel jobs! Can we use one self-hosted CI/CD (agents) to store the artifacts and deploy to all 4 servers or do we need to buy 1 (free) and 3 ($15 per extra self-hosted)?
You only need to pay for the number of CI/CD jobs you will use. If you're okay having all your jobs for the four environments (Dev, Stage, Test, Prod) waiting in a queue for that single job then you do not need to pay for anything.
However, be aware that a long running job to Stage (for example) will block all other jobs until it completes. And if you use multiple jobs per pipeline you will get no parallelism benefits unless you pay for parallel jobs.
You can also have multiple self-hosted agents, but only one of them will be able to run a job at a time, unless you pay for parallel jobs.
UPDATE: Highlighting Matt's comment below: If you have MSDN or Visual Studio Enterprise subscribers as registered users in the DevOps organization, you'll get one free parallel job for each of those users.