In our pipeline we use three embedded boards for automatic unit test execution. These boards are liste under Lockable resources in Jenkins config as Board_1, Board_2, Board_3. All get the same label 'imx8'

The boards get locked on start of every corresponding build. If there are 3 concurrent builds, the last one to start will report:
Found 0 available resource(s). Waiting for correct amount: 1.
At this time only 2 boards are used and the third never gets allocated.

Call for locking the boards is:

 
lock(label: imx8, quantity: 1, variable: 'LOCK_NAME') {
    ...
}