I'm almost sure that this worked in previous releases: changing the number of
executors on the master in Hudson via the UI needs a restart of Hudson to be
recognized.

Usecase: we have several parallel database clone jobs running, that almost take
no processor time. If all executors are blocked by those jobs we would like to
increase the number of executors dynamically, allowing our compile job get running.

(If this is intended then a message "please restart Hudson" would improve the
usability.)