First of all, the Templars that appear on the cutscene are not the exact numbers of Templars that are at the entrance anymore than the mages that appeared in the cutscene were all that there was of the Circle's fighting force.
It's meant to convey a situation, in this case, Hawke is outnumbered and surrounded by Templars; not "Ok, I'm counting 25 Templars, we can totally do this, you guys."
Second, the Circle is not a couple of corridors. There was way more Circle than what we saw.
Seriously, don't try to use the numbers that appear in cutscenes as evidence of the size of a population. Otherwise, there are more criminals than law abiding citizens in Kirkwall and the elven population is a grand total of 15.
And it works both ways given that the entrance to the gallows in which we fight Meredith is small, at most hundred people fit there.
What.
Magneto managed to took out armies several times, he struggled but even took them out when they used plastic. I did say it depends on the situation therefore its hard, not impossible. DA is not exactly reality.