The number of build agents that you can use will depend on a number of factors, including the resources available on your build servers, the demands of your build process, and the number of parallel builds that you want to run.
In general, it is a good idea to have enough build agents to ensure that builds are completed in a timely manner, while also ensuring that you have sufficient resources available to support your build process and release orchestration. This may involve using a combination of local build agents and cloud-based build agents, depending on your needs.
Some continuous integration (CI) tools, such as TeamCity, allow you to scale up the number of build agents as needed, either by adding more agents to your build pool or by using a cloud-based agent pool. This can be useful for handling peak workloads or for running parallel builds.