Zones are sized to reduce the number of players that can see each other, in order to reduce outgoing bandwidth consumption.
How many zones per server depends on your game. If you use Everquest size zones on an Everquest style game, considering it was working a decade ago, you will be able to get a lot of zones on a modern server. There is no good reason to use one server per zone in this case, but you may find you need more than one server for all of them.
How many zones per server depends on your game. If you use Everquest size zones on an Everquest style game, considering it was working a decade ago, you will be able to get a lot of zones on a modern server. There is no good reason to use one server per zone in this case, but you may find you need more than one server for all of them.