Let's say 20 million humans live in Thedas, like Late Medieval Europe.
And like 1/20 humans are born mages.
That means 1,000,000 mages are around.
However, due to limited offsprings to be made with mages due to being discouraged in the Circle and even outside Circle apostates are mainly isolated, density of mages is very low and most reside in Tevinter Imperium.
Let's give random number and 800,000 mages live in Tevinter Imperium, since it's very encouraging to have mages in your bloodline and many apostates flee there. So 200,000 in rest of Andrestian kingdoms and let's say every 1/10 of them are apostates. So 180,000 live in Circle and since there are 12 of them, Circles averagely house 15,000 mages but number should vary per each Circle.
And Templar's should outnumber a little bit mages, so let's say there's 200,000. Around 16,600 per Circle.
So, this numbers are realistic though maybe a bit too large, especially since I didn't count other races. But should be close to real numbers.
Quite a lot for sizable war too, forcing kingdoms to take side or go against both since their lands will be battlegrounds. As well, some nobles can take advantage of situation to attempt taking kingdom for themselves or for independence, by allying with one of the factions.
So yeah, a giant mess and even more so required to reach a compromising solution as both sides will have significant numbers, resources and lot's of motivation to fight til the end. And this war will only be behind Blights in caused casualties, if it get's prolonged.
So either a total victory of one of the sides or they'll have to be forced to compromising new solution.