The safety in numbers effect as it relates to cycling is that the more bicycles on the road the safer each cyclist is. More info here. This effect was recently seen clearly in a study of bike crashes in Minneapolis. As more and more people commuted by bicycle, the ratio of bike crashes to bicycle commuters dropped.
This is really encouraging information, because it means that one of the best ways to increase cyclist safety is to increase the amount of cyclists! The more people riding their bikes the safer the roads will become!