No, you need to buy only ES 50 web sites license and install it to DNP Enterprise Server.
DNP Server has designed so it can works without a license on the SAME machine with Enterprise Server. You just add the local DNP Server in DNP and it works.
But if you want to control additional DNP Servers (on other machines) you need to buy DNP Server licenses for them.
Please, let me know if it's clear.