Both questions are really hard to answer. You can only estimate how often this happens.
There is no monitoring system, which constantly observes the entire sky. There is an unknown number of asteroids and comets in the solar system. Right now, the rate of newly discovered objects increases exponentially (due to better and better technology). Today's number of uniquely identified objects is in the range of half a million to one million objects (620284 as of July 22 2013). They are listed by the Minor Planet Center. While there actually are estimated values of the total amount of objects in our solar system, nobody can be sure about it at the current state of science, technology and monitoring. There are some attempts like observing and counting present impacts and dating existing craters on the Moon, but this only helps for the class of objects, which intersect Earth's orbit around the sun. For instance, it excludes objects in the asteroid belt or Kuiper belt.
Keeping in mind those numbers, inter-asteroid / asteroid-comet / inter-comet collisions are extremely rare. They are in fact so rare, that there is basically no statistics about it. It is hard or near impossible to put a number of that.
However, there are attempts to predict collisions. Keeping in mind that orbital parameters of small and distant objects can only be determined with some uncertainty, the idea is to go through the database of known objects and predict when a pair of objects comes into a certain proximity. This gives candidates for collision events. They are rather interesting for researchers, who are studying the structure or chemical composition of such objects. Another however, the amount of collisions actually happening after such a prediction is still incredibly low.
If you are interested in such events, it is a good idea to set a number of Google alerts on some relevant key words and follow as many astronomers mailing lists as possible. This may sound like stupid advice, but it is as good as it can get. Sometimes, there are announcements. You just need to find them at the right time.