Isabella May would be treated as one name, unless sometimes she went as Anne Isabella or Anne May. The general rule of thumb is that if it is part of the proper name, it can go in the appropriate field. The one name per field is there to remind you to only put the primary name in that field and to save nicknames for the nickname field, so instead of reading Anne Annie Booboo Isabella Izzy May, it reads Anne Isabella May, aka Annie, Booboo, Izzy.
Another example is with traditional Spanish surnames, they generally combine the mother's name with the father's, something like Arnaz y Paolo. Since both Arnaz and Paolo are the surname, it would be inappropriate to separate them.