This wiki contains spoilers for all stories of Critical Role. This includes the story for unaired episodes of The Legend of Vox Machina, as it's based on the first campaign of Critical Role from 2015-2017.
Since there were over 1,000 deaths when this garrison fell to the Kryn Dynasty, it must have held at least this many troops.
The Dwendalian Empire attempted to attack the Dynasty-held garrison with three legions after sending the Scourgers to strategically assassinate members of the Dynasty as a distraction, but Jester overheard King Dwendal give these orders while she was Scrying on Vence Nuthaleus, and the Mighty Nein warned the Kryn of the impending assault. Eight days later, Leylas Kryn thanked the Mighty Nein because their warning saved many lives on the Dynasty side, and Dairon reported that the unsuccessful attempts to regain the garrison cost many lives on the Empire side. More than a month later, the Empire did retake the garrison, at the cost of many more lives.
During the Kryn occupation in 835 and 836 PD, the Ashguard Garrison had a population of 5,720, of which 74% were dark elves (drow) while the other 26% were other races. It is unknown what the demographic makeup of the garrison is after the Empire retook it.