Chicago has a special relationship with linebackers. Some of the true Chicago legends played linebacker--George, Butkus, and Urlacher to name the most obvious, but the list is long. Once upon a time, off-the-ball linebackers were the heart of a defense, and they helped to define a defense in a way that currently falls to edge rushers and hybrid ‘safety-backer’ types.
The actual number of linebackers who see the field varies dramatically, regardless of whether or not the system is designated a 3-4 or a 4-3, because as I mentioned in the Interior Linemen article, the concept of a Base Defense is more or less antiquated.