I had a 10x target scope on my MS rifle and would regularly shot coyote sized Ram targets offhand at 500 meters.
My 12x never went over 300 yards on my target rifle.
We used receiver sights to shoot target to 600 meters.
So your question needs more refinement to give a better answer.
Things like the quality of the glass, reticle, turrets, size of the target, your ability to read wind, your shooting ability all effect how far you can shoot with a scope. Power is one of the least limiting factor.
A good 12x scope in the hands of a good shooter will be able to make hits at a much farther distance than a mediocre shoot with a cheap 24x or even 32x scope