DMXGetScreenCount man page
DMXGetScreenCount — determine number of back-end screens
Bool DMXGetScreenCount(Display *dpy, int *screen_count);
DMXGetScreenCount() returns the number of back-end screens that a Xdmx(1) server controls. Since a DMX screen usually fills all of the available area on a back-end server, there is usually a one-to-one correspondence between DMX screens and backend servers. However, it is also possible for a DMX screen to cover only part of the available area on a back-end server, and for more than one DMX screens to occupy different parts of the visible area on the same back-end server.
A DMX screen may be managed as a regular X screen in the Xdmx(1) server or may be joined with other DMX screens using Xinerama.
DMXGetScreenCount() sets screen_count and returns True unless there is a protocol error.
If Xinerama is active, only one logical screen will be available to clients of the Xdmx(1) server. However, this call will accurately reflect the number of composite back-end screens that the Xdmx(1) server is using.
DMXGetScreenAttributes(3), DMXChangeScreensAttributes(3), DMX(3), Xdmx(1)
DMX(3), DMXChangeScreensAttributes(3), DMXGetScreenAttributes(3), DMXGetWindowAttributes(3).