Lets say we have this bacterial culture in a jelly agar and we want to see how many bacteria would x quantity of a antibiotic kill..Is there a way to get that info?

For example, I have a e.coli culture and want to try amicillin on it and want to know (in % maybe?) how many were killed...

Basically you would simply count the titer. Either count the whole (live and dead) titer and then count only the surviving part (either usind live/dead stain or plating and counting the survivors). You could also simply split a culture and treat one portion and compare the live titer in each case.

you could also count CFUs on an agar plate from before and after treatment. This will give you a relative measure.

That's what I actually meant with live titer...