By default the Remote Desktop protocol allows up to 16 monitors per session. You can restrict that to any number from 1 to 16 in two ways.
Group Policy
Open the Local Group Policy Editor (gpedit.msc) or a domain GPO and enable Limit number of monitors:
Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Remote Session Environment
Set the number of monitors you want to allow. It is enforced on the host, so a client that connects with more simply gets the capped number.
Windows Server 2008 R2 and earlier
Use the Limit maximum number of monitors per session setting in the Remote Desktop Session Host Configuration tool (tsconfig.msc).
For the maximum resolution limit and the client-side .rdp options, see how to limit Remote Desktop (RDP) monitors and resolution on Windows Server.