Microsoft Dynamics Nav Licencse Issue

Hi,

I created an application to take orders from a website from our Customers (1500 customers) which are existed in navision database and it uses Navision webservices to check stock level and to place final order. for remaining item/customer lookup etc it uses sql server (sql server 2012 core Licencse been used) .

In this scenario Do I need Licence for Navision webservice? If needed How many licences and what licencses do i need to buy.

Please let me know ASAP , Thanks in Advance

Regards

MOULI KAKU