Resolution
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
This article is part of our Knowledge Base and is provided for guidance-based purposes only. The solutions or workarounds described here are not officially supported by CloudBees and may not be applicable in all environments. Use at your own discretion, and test changes in a safe environment before applying them to production systems.