poles and zeros of control system
No, the number of poles does not need to be equal to the number of zeros. One could be greater/lesser than the other. For instance, a differentiator or integrator could have an unequal number of poles and zeros. It is simply the nature of the control system that determines this relation. 

The assumption that zeros lie at infinity is done only in the case of root locus plots. These zeros are only introduced to complete the definition of the root locus equation, but do not actually occur when you take a look at the transfer function.