The zone number is automatically assigned to the devices according to the order in which they are added (it is NOT modifiable). That is, the first device we add is zone 1, the second is zone 2 and so on.
Let's imagine the situation of removing the device that occupied zone 2 and that we currently have up to zone 8 registered. If we delete the one in zone 2, the next device we add will occupy zone 2 and not zone 9. The next device will follow the order from the last one though, occupying zone 9 and so on.
Therefore, it requires a little effort from the installer to keep track of this as tall devices are registered. As an alternative option to find out which zones they occupy, we can enable 'Groups' mode. This way, we will be shown the devices' ID on their respective configuration menus.
If we don't have this option enabled, we can always turn it on, put all the devices in a group to see their zone number and then disable this mode. As you can see in the next screenshot, ALL devices occupy a zone in Ajax.