Matrix projects are consuming one extra executor
- CloudBees Jenkins Enterprise
- Matrix project plugin
Matrix projects execute the axis using Flyweight executors so Jenkins doesn’t use a normal executor, also referred to as Heavyweight executor for this. Notice, that one of the apparent executors doesn’t have any number assigned. so it is not consuming any executor.
For a more detailed explanation about the differences between Flyweight and Heavyweight executors, please check Pipeline - Difference between Flyweight and Heavyweight Executors