How do I know how many more defenders are there in a city?
Solution 1:
As far as I'm aware you can't take a city with ranged units, so they are probably just wasting their attacks at this point.
Cities do also heal (about 10 to 20 points a turn) each turn so in between your attacks the city is probably recovering.
You'll need to bring a non ranged unit in to make an attack, at which point they should take the city.