It is often said that the radius of a cation is less than parent atom due to decrease in number of shells. But my view point is that the number of shells in an atom are constant I.e., infinite. They can be empty but they are there. So is this correct. Please give reason also.

The shells are not something like rigid circles... It is just a path of electrons in an atom.. It exists only if there are electrons in that shell... Don`t think that it is a series of concentric circles which forms shell.. So if that electron leaves the shell that shell will not exist... SO the radius decreases... Hope U like it