Is there a canonical answer to this? I remember seeing “70” somewhere, but wasn’t sure where that had come from or if I’m misremembering. I know Midnight’s army in Fall 1152 had about 100 mice and it seemed like a genuine threat to Lockhaven.
I have no idea, honestly. I’d be interested in an answer, though, because 70 seems incredibly low.
Another thread, in this section of the forums, places an estimate on the number of mice living in the Territories at around 20 thousand. To get this number, I believe the poster took into account both the size of the Territories, along with data concerning population density of mice in the real world. Seems like a pretty good number, if you ask me. Would civilized mice have a higher population density than wild mice? Maybe? I dunno. I suspect that it’s probably about the same, just much more condensed and centralized in towns and cities.
If you use 20 thousand mice as a base population for the Territories, and you assume that even 1 out of every hundred mice swear an oath to protect the Territories (which I suspect is pretty conservative, to be honest), then the Guard is still around 200 mice strong.
200 is to high for my taste. Real mice breed much more than Mouse Guard mice do i think.
That’s probably true, yeah. What do you think would be more appropriate as a base number? Ten-thousand? That would place the total number of Guardsmice around a hundred; again, assuming that 1 in every hundred mice swear an oath (which, again, seems a bit conservative to me). I could personally see the Guard as high as 150-200 mice, I think. They don’t all have to be out on patrol, I suppose. Lockhaven is likely full of Guardsmice that are permanently stationed there, as craftsmen, archivists, and the like. Still, I have no idea how many mice realistically make up the Guard, and I suspect that until we can agree upon the total number of mice residing in the Territories, we’re never going to figure it out.
I still think that serving the guard is a rare choice for a mouse. 1 in 100 could be right. In my world the guard is understocked most times and not allways able to send out mice as fast as Gwendolyn would like.
I’m kind of surprised Mr. Petersen hasn’t dropped Canon on us, yet. Can someone in BWHQ ping him and bring this thread to his attention? Seems kind of silly to speculate when the authority on the setting posts here occasionally. 
I read ~70 guard mice in the book somewhere. I would assume that they are supported by tons of logistics oriented mice that do not count as guard.