This higly depends what services you are going to install and use. And with those services, it highly depends on configuration of those services.
If you want to use the Firewall/Gateway modules and the proxy module, a lot depends on how many rules you need to implement. These can take a lot of resources when there are many simultaneous connections.
On the other hand, a windows server with similar services will at least need the same amount of resources, and my gut feeling says, it will take more. (I am curious what you find!)
NethServer does scale well. If you find you need more resources you can easily add another server to your domain. With the last iteration of Samba4 account provider you can join an existing domain. IMO it is a good idea to have 2 servers anyway. If 1 server fails, you have a 2nd server available.