As I understand it how many there are depends on how many people want to be in that map. There's always a minimum of 6 (one for each platform and region - PC EU, PC NA, Xbox EU Xbox NA, PS EU, PS NA) but each server could have more if more people want to go there. So a map like Craglorn which is often quite busy (at least in the town) will probably have a few instances on each server, but somewhere which is currently quiet (like the Gold Coast) might only have 1 per server.

Annoyingly there's no way to see if there are any more instances for the map you're in or a way to move between them easily. Sometimes logging out and back in will do it, but there's no guarantee.

This is also one reason I sometimes wish this game had a group finder like Guild Wars 2 where instead of just joining a queue and being dropped into a random group with space you can use it to choose a group. People who want to form a group write a short description of what they're looking for and then anyone looking to join a group can choose one which is right for them. It's basically like spamming zone chat but everyone on your server can see it even if they're not in the same map you are and you only have to write the message once. (Also it means no one has to spam zone chat so it can be used for other things.)