Build Executor display number is not consistent with the number is provided by EXECUTOR_NUMBER variable.
Display number should always by EXECUTOR_NUMBER+1 but it randomly shows different values.

When using 4-5 executors per node (master included) seemed to be easier to replicate but not 100%.

It seems to only be an issue on the display side.
When using EXECUTOR_NUMBER to separate folders of maven repository artifacts, to build in parallel, the value is very important to avoid "cross contamination".
We also use this variable to avoid constant download artifacts on every build, by mounting /var/lib/jenkins/.m2/repository/EXECUTOR_NUMBER inside a container.

Jenkinsfile:

node('master') {
    stage('on master') {
        echo EXECUTOR_NUMBER
        sleep 30
        sh 'echo ${EXECUTOR_NUMBER}'
    }
}

node('docker') {
    stage('on slave') {
        echo EXECUTOR_NUMBER
        sleep 30
        sh 'echo ${EXECUTOR_NUMBER}'
    }
}

Not sure if there is some way to get the Build Executor display number, so to compare, I took a screenshot from the nodes page, during the build.