What factors determine how many external screens (monitors/displays) a laptop can support (with specific requirements)?

Question

What factors determine how many external screens (monitors/displays) a laptop can support?

For this specific question, let's define several parameters:

• we are working with modern (2023) technologies
• minimum resolution of 1920x1080 pixels per screen (1080p)
• each screen needs to display its own content (no mirroring)
• each screen requires graphics acceleration (to play video, perform 3D CAD manipulations, etc.)
• allow dragging windows between screens
• strongly prefer that the laptop's integrated screen also be used in conjunction with the external screens, but this is not a hard hard requirement

Answer 1

It is entirely down to how many display outputs the "primary" graphics chip has. And I don't mean the Nvidia chip in laptops that have optimus, I mean the actual chip that is connected to the CPU and actually has display outputs wired up.

You have to go to the manufacturer of that chip (usually Intel or AMD) and find out how many displays it can support. Then you have to go to the manufacturer of the laptop and ask them how many of those displays they bothered to wire up.

Note that I do not mean Nvidia Optimus laptops where the secondary GPU is essentially a headless graphics card that renders frames and then copies the display buffer over to the Intel graphics. In those laptops the Nvidia graphics has nothing to do with actual displays. There may well be newer Nvidia laptops which are the primary display adaptor but for years these were uncommon as it allowed the designer to disable the Nvidia graphics to save power and use just the low power Intel graphics.

For example the Intel Iris Xe supports 3 displays, but the laptop designer might only wire up one to the built-in display and one more to the USB C or HDMI output and so leave one "dead" output.

The only people who can really tell you what they bothered to wire up, and thus tell you how many simultaneous displays you can have, is the machine manufacturer.

Answer 2

Practically - the GPU and how many RAMDACs it has

Modern GPUs, even integrated ought to have enough to support every display it has. Of course, it depends on 'how' the PC is designed, and how your system is set up.

minimum resolution of 1920x1080 pixels per screen (1080p)

Which sort of makes things easier, cause with the right ram setup, even a 7-8 year old low end system can power 2-3 displays of that resolution. My old 3050 can't play 4k video but it can output 4k with dual channel ram set up. There should be nothing in 2023 that can't output more than one FHD screen.

each screen requires graphics acceleration (to play video, perform 3D CAD manipulations, etc.)

That's a function of the GPU - you need at least one powerful enough. On multi GPU systems there may be ways to pick which GPU to run.

Also consider outputs - how many HDMI and DP (rare on laptops) ports you have, and more 'interestingly' - what your USB Ports are. Some laptops have USB C ports that act as DP out. I currently have a dock, that, for example will let me break out a video capable USB C port to 2 HDMI and a DP port.

If you have USB 4 or Thunderbolt 3/4 ports, you could use a dock that connects to more monitors or even a full desktop GPU in a dock. Some particularly adventurous folks also have use an onboard mini PCIe, m.2 or expresscard slot with a breakout box like the GDC Beast to run a desktop

You might also be able to use Multi Stream Transport enabled monitors in some setups to use one video out for many monitors. You'd need to check for support.

You probably could test the maximum supported video outs with the cheap 'dummy' HDMI dongles used for some headless streaming or bitcoin mining operations.