There's no feature to do that at this time. You could e.g. script this by making multiple global_prefs_override.xml files and shortcut button to alternate the file settings and read in the preferences. Cumbersome. Maybe someone really smart already did this with a little detection utility or reading the boinctray.exe output on Windows, to automate this.

The only way to currently limit cores would be to set the memory during use/idle in such a way that X tasks fit in memory when system in use and Y tasks when idle. It would require a single science to be selected of stable size, e.g. HCC1. Set 400MB allowed during use and max when idle. BOINC would then suspend any task that does not fit in the 400MB. You'd have to find out how much the science actually uses.