x-display-screens

x-display-screens is a built-in function in `C source code'.

(x-display-screens &optional TERMINAL)

Return the number of screens on the X server of display TERMINAL.
The optional argument TERMINAL specifies which display to ask about.
TERMINAL should be a terminal object, a frame or a display name (a string).
If omitted or nil, that stands for the selected frame's display.