Conceptual Physics (12th Edition)

The maximum would be when both speeds equal c. The velocities can never be larger than c, so the fraction can never exceed 1. When the speeds both equal c, then $\frac{v_{1}v_{2}}{c^{2}}=\frac{c \times c}{c^{2}} =1$. The minimum would be when either speed equals zero. When one of the speeds is zero, then $\frac{v_{1}v_{2}}{c^{2}}=0$. This is discussed on page 673.