Depends on your environment and the level of security you are looking for. You need to evaluate your network, the type of work that is done on it, and the likelihood of a virus infection. Taking that into consideration, factor in the likelihood of the same virus infection affecting multiple PCs on your network within a given amount of time. I could give you a number, but 10 PCs on a network of 200 is far different from 10 PCs on a network of 2000.