SQL Server - Service User Accounts and Licensing?
Basically User CALs are for people, not for Service Accounts.  In the User CAL model you need a CAL for each person whether they directly access SQL Server from their account or they access SQL Server through some Service Account.

This is a CAL per each and every person that uses the SQL Server, not a CAL per simultaneous user. However, your CALs will allow you to access as many SQL Servers that use CALs as you can reach within your organization.

Edit: There are also Device CALs for when several people share a device at different times.  Read about CALS at:  

https://www.microsoft.com/licensing/about-licensing/client-access-license.aspx

Regarding Service Accounts on the SQL Server I cannot find my old link on the subject,  but perhaps this post will help you:

http://blogs.technet.com/b/uspartner_ts2team/archive/2013/11/05/administrative-amp-service-account-cals.aspx

See the sentence that says


  And for system service accounts – no CAL is required.


Therefore, since Service Accounts are not people, you can use as many service accounts as you can make use of: SQL Server, SQL Agent, Full Text, and so on.