By default, RDP only allows you to have 2 people login remotely at the same time to work on the server (with Remote Desktop for Administration setting). You can also have another person logon at the console so that makes 3 people can work on the server at the same time.
Here's a little known tip: You can emulate the console login so you don't have to be right in front of the console to login as the 3rd person and have 3 people RDPing at the same time.
To emulate console login, use the following Command line syntax:
mstsc /v:servernameorip /console
You can also shorten /console to /c to save some typing.
If you have to RDP into your servers, it's a good security practice to always try to VPN in and then RDP to the machines if you can. Otherwise, if firewall or router permits, then only allows RPD from known location (your office, your home IPs...). And as the last resort, open RDP port for access from any where.