The seven thains of Morgoth

[Keeper of Dol Guldur]

And to bring up the fact that when Ungoliant attacked their master, the whole of them came (it said something like, "and out from Angband came the balrogs), and if there were more then seven, Ungoliant wouldn't have had time to flee, since they went with winged speed, probably faster than the spider, and would have probably gone after her if they had power enough. So it is easy to see a small number of ultimate creatures, which almost never faltered, leading Morgoth's hosts. Seven, with lesser demons not unlike your Boldogs under their control. After all, we know Balrogs weren't the only maiar under Morgoth's control, he had Sauron and the Fell Rider who took elves in the darkness (of whom the light of Anor (like gandalf) shone in his dreadful eyes.)