You always go to limits and say the circle is full of blood mages based on the number we fight until we reach Meredith. By that assumption yes we do kill a chunk of Templars, how else you justify Hawke having a full tour of the circle. All templars who were not at the entrance were naturally killed.
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.