Are these the number of the processors used for boot time or the cpu cores? Also, what's the default value and what would happen if I activate the max number of processors on the drop down list?
Default is the box Unchecked (means use all processors and threads), nothing bad will happen if you change it.
I have read post on both sides of this question, some say using all will boot faster, some say no. All I can say is try it for yourself and form your own opinion about boot performance based on your findings and hardware configuration.
Conclusion: In Vista and W7 it uses all cores by default to boot and run, so this setting is more for disabling cores/processors not only at boot time, but disabled period.
I can detect no difference if i tick the box and set it to four, compared to the box being unticked.
If you experiment and need To restore all processors and threads, uncheck the box and reboot.
Here is a screenshot of the taskmanager with "number of processors" the tickbox unchecked, it shows all four cores as it should for my processor.
Now I set it to 2 processors and rebooted, (did not notice a significant difference in boot time)
Taskman now show only 2 cores operational