Adding a 2016 Domain Controller and Licensing

Hi Experts,

We currently have 3 domain controllers and have a 2008 r2 Active Directory Domain.

I would like to upgrade our domain to at least 2012 r2 but would also like to add a 2016 DC to our domain.

We currently have 40 CAL licenses for what we have now, do we need to purchase additional licensing to add the 2016 server as a DC or anything else or just the 2016 server license?

Thank you,

Karen