As @NickD said in a comment, display-pixel-width uses the current display/frame by default, but you can provide the one you want to consider as an argument.
There is also function display-monitor-attributes-list, which gives you quite a lot of info. C-h f tells us:
display-monitor-attributes-list is a compiled Lisp function in
frame.el.
(display-monitor-attributes-list &optional DISPLAY)
Return a list of physical monitor attributes on DISPLAY.
DISPLAY can be a display name, a terminal name, or a frame.
If DISPLAY is omitted or nil, it defaults to the selected frame's display.
Each element of the list represents the attributes of a physical
monitor. The first element corresponds to the primary monitor.
The attributes for a physical monitor are represented as an alist
of attribute keys and values as follows:
geometry -- Position and size in pixels in the form of (X Y WIDTH HEIGHT)
workarea -- Position and size of the work area in pixels in the form of (X Y WIDTH HEIGHT)
mm-size -- Width and height in millimeters in the form of (WIDTH HEIGHT)
frames -- List of frames dominated by the physical monitor
name (*) -- Name of the physical monitor as a string
source (*) -- Source of multi-monitor information as a string
where X, Y, WIDTH, and HEIGHT are integers. X and Y are coordinates
of the top-left corner, and might be negative for monitors other than
the primary one. Keys labeled with (*) are optional.
The "work area" is a measure of the "usable" display space.
It may be less than the total screen size, owing to space taken up
by window manager features (docks, taskbars, etc.). The precise
details depend on the platform and environment.
The source attribute describes the source from which the information
was obtained. On X, this may be one of: "Gdk", "XRandr", "Xinerama",
or "fallback".
A frame is dominated by a physical monitor when either the
largest area of the frame resides in the monitor, or the monitor
is the closest to the frame if the frame does not intersect any
physical monitors. Every (non-tooltip) frame (including invisible ones)
in a graphical display is dominated by exactly one physical
monitor at a time, though it can span multiple (or no) physical
monitors.
