Calculating the number of gates I need..??

True as Harald says about Migration.

But you may decide your pins early and may not need any extra pins later but just extra internal resource.

For this end some devices are manufactured as a set of different sizes but with same footprint, the largest device will have the full set of pins, smaller devices will be subset of the main pinout. Then you can start buying largest device of a set and then migrate down if necessary provided you have used the correct subset of pins on your board.