You can set up Cloud Build from the Unity Developer Dashboard. See Setting up your project for Unity Services.
There are two types of build caps that you can set up to manage your spending.
Soft cap You can set a dollar amount where you are notified by email when reaching the end of your build minutes cap. You can continue to build.
Note: Soft caps are disabled by default.
Hard cap You can set a limit not to spend more than a certain amount per month. By default, the hard cap is set to be $0 (you can change this value) so you don’t incur additional changes.
Hard caps do not kick in for ongoing builds. For example, if you reach a cap midway through a build, Cloud Build doesn’t cancel the build. We will let the build finish and charge you for whatever part of that build exceeds the hard cap. No new builds are allowed until you increase or disable the hard cap.
Note: We never want to cause unnecessary downtime for your sites, so we won’t stop builds when you’ve hit your limit. This means that there is no guarantee that you will not incur charges even when the $0 cap is set.
If you reach the soft or hard cap, Unity sends you an email notification that you’ve exceeded your specified cap. You can also monitor your build minute usage in the Unity Developer Dashboard.
In the Unity Developer Dashboard:
Note: If you exceed your cap while a build is in progress, the build completes, but you are charged for the excess build minutes.