There are only eleven breakable monitors (exactly as many as you need for the achievement). There are several other unbreakable monitors, but I don't have an exact count for those.
The first ten breakable monitors are pretty straightforward once you realize what you need to do, but the last one is quite tricky even once you figure it out. If you're short only one, that's probably it. If you're short more than one, go back through, looking carefully at each monitor that you can't figure out how to break. If there's nowhere to place a portal that would allow you to fling yourself or a crate at it, maybe there's something else in the level which could damage a monitor?
If you're running into real trouble and getting frustrated, look at DMA57361's answer to a question about the achievement. It uses spoiler tags to protect the details, but can also be used as a list of which chambers/monitors you should be looking at.
