Veeeeery difficult to say. The greatest concentration was... well, obviously it was during the Spring of Arda, when the entire host of the Valar lived in Almaren in Middle-earth! After that, however many Maiar Melkor had under his dominion remained there until the Siege of Utumno, and some (Sauron, the Balrogs, presumably others) lingered much longer.
During the First Age, we have at least three named Maiar hanging out in Beleriand: Sauron, Melian, and Gothmog Lord of Balrogs. Durin's Bane was also present, and the Balrog killed by Glorfindel.
But what about Lungorthin, often poetically translated as the Balrog of White Flame? He's mentioned exactly once, in an early draft of the Silmarillion. Do you consider him to still exist, since he wasn't explicitly removed?
And how many balrogs were there, anyway? Tolkien seems to have settled on '3 or at the most 7', but which is it? If three, then we have the bizarre situation where two of the three were killed in Gondolin, leaving Durin's Bane all by itself.
Moving on from the balrogs... do Osse and Uinen count? They definitely hung out in the waters around Middle-earth, though not on land (not really their thing). How about the dragons? I'm pretty sure at one point Tolkien considered the possibility that they were mechanisms 'powered' by Maiar. How about the werewolves? The vampires - Thuringwethil
was a shapeshifter, does that make her a Maia? What about the Great Eagles? Or Huan the Talking Dog? Heck, at one point I think Tolkien thought about designating certain orcs as Maiar!
Even worse - Ungoliant? Tolkien certainly had her as a Maia at one point. And if she is, then... Bombadil? The River-Woman, mother of Goldberry? Do the water-spirits under Ulmo who live in the rivers of Middle-earth still exist (as Maiar, since all the earlier 'spirits' ended up as Maiar in Tolkien's later versions)? Are they identical with the mermaids from Tolkien's early writings, and if so, does that mean the sylphs and sprites also still exist?
You see the problem. While we could certainly make lists of
confirmed Maiar present in each Age after the rising of the Sun (1st: Sauron, Melian, and 3-7 balrogs; 2nd: Sauron, Durin's Bane, potentially the Blue Wizards if their arrival with Glorfindel in the Second Age is accepted; 3rd: Sauron, Durin's Bane, 5 Istari; 4th: Radagast, maybe the Blue Wizards), making a list of
all the Maiar present requires a massive number of personal decisions about what you accept as canon.
hS