Hi Gianluca,
You are right, in oVirt 4.2 we removed the 'Num Of IO Threads' text field from the UI because it was decided that one IO thread is the best configuration from performance aspects for most cases (according to performance testings). We wanted to keep the UI simpler to use.
So in case IO threads is enabled via Webadmin UI, only one IO thread is always created.
For REST API the configuration remains unchanged in 4.2, so you can still set the number of IO threads to be >= 1 if you want for some reason.
Regards,
Sharon