Windows by default allows two
RDS RDP* connections out of the box, and they are known as "administrative connections" regardless of if the user account is an administrator or not. If you need more remote connections you need to research RDS which requires it's own set of user connection licenses in addition to CALs.
RDS can allow for a number of simultaneous connections that is only limited by your bandwidth and server hardware. To do so, you will need a multi-server infrastructure of at least two machines to host the various RDS roles as well as at least one domain controller. TS Gateway and RDS Session Hosts shouldn't be included on a domain controller, but you can potentially get some of the services to work on a domain controller. It's a terrible hack that will probably break every time Windows applies patches, or it might be the opposite scenario where the hack will break certain Windows patches.
*(I edited this to clear up some of the confusion. The term "RDS" refers to a larger suite of products that implements virtual desktops and RemoteApp. RDP is the protocol and the term "Remote Desktop Connection" [RDC] is typically used to describe the generic Remote Desktop Connections that administrators would typically use to work on a server directly.)