License to connect custom applications and Business central cloud

Maybe there is already a thread with this information but I have not found it.
We have an organization with many users geographically dispersed and accessing the applications. I expose the case:
1) we have about 500 users that use a production module and a purchase order module, both custom applications 
2) We want to implement dynamics businesscentral in the cloud Azure
3) We want to connect custom applications and BC for information exchange

The question / doubt is:

1- do we need to buy 500 user licenses? 

2- Is only one license needed for the connection of both platforms? Wich one?

3- Or is no license necessary?

thanks for your help and answers