[denethorthefirst]

Tolkien never gave us a number regarding the Maiar, but I always imagined that there must be a lot of them, obviously a lot more than the Valar. Including Melkor there are 15 Valar. So how many "helpers" would a single Vala have? At least a few dozen I imagine, probably more, because the responsibilities and labours (during the Shaping of Arda) were just so enormous. So there must be at least over 1000, maybe even 3000 Maiar. That is just my head-canon. Of course I have no quotes to back that up, its just a number I pulled out of thin air.

In the early days, during the Shaping of Arda, most of them lived in Arda (Almaren), but after the Destruction of the Lamps almost all of them fled to Valinor and only the Umaiar remained in Middle-Earth. In biblical cosmology (during the War in Heaven) a third of all angels sided with Satan. Tolkien obviously drew a lot of inspiration from the bible, so I think its likely that the numbers would be comparable and at least a third of all the Maiar sided with Melkor during his rebellion. So the Umaiar numbered at least in the hundreds. Sauron and the Balrogs (all together 8 Umaia) were obviously very powerful and at the very top of Melkors power structure, but they probably made up maybe only 1% of all the Umaiar, like the the upper crust of the aristocracy in human societies. The majority of the Umaiar probably consisted of minor spirits and Boldog-like creatures that were maybe already completely incarnate by the time of the Awakening of the Elves. A lot of them probably "died" during the War of Powers and the later War of Wrath and were not able to re-incarnate.

By the time of the Second Age most of the (incarnate) Umaiar were probably "dead" (for all intents and purposes). If even a being as powerful as a Balrog was so afraid that he hid himself away for thousands of years, how afraid must the other surviving Umaiar have been? Again, just my head-canon, but I guess that maybe at best only a few dozen of them survived and I do not think that a lot of them served under Sauron in the Second Age. At least not a single Balrog did. Maybe Sauron pressed some of the surviving Boldogs (and other weaker incarnated Umaia) into his service or used them for breeding.

In the Third Age you obviously have the five Istari, Durins Bane and Sauron. Those are the only mentioned Maiar. Again, just my head-canon, but because I believe their number was so large in the beginning, I guess that there must have been more survivors who simply hid themselves away and/or simply showed no interest in the War and/or were not living in the West (after all, Arda is a very big place), maybe even some good natured Maiar. Radagast or Bombadil-like figures, who rejected the bliss of Valinor and favored Middle-Earth. After all, if two of the Maiar (Melian and Radagast) fell in love with Middle-Earth and its creatures, then there must be more. Melians and Radagasts actions don´t strike me as that excentric or unbelievable. If two Maia harbored feelings like that towards Middle-Earth then there must have been more who felt the same and stayed.