How to use multiple displays ?
There is no limit in Millumin about displays, the only limit is the hardware : in brief, this is how many displays you can physically connect to your computer. If you plan to buy an Apple Silicon machine, be sure to check this article to learn about physical displays limitation : Recommendations .
Also, if you need an adapater (for example from Thunderbolt to HDMI), It is better to use an active one. For a better understanding about active VS passive adapters, you can watch this video.
Lastly, if you need more video outputs than your computer can offer, please read the sections below.
Splitters / Wall Controllers
A video splitter is copying the same image from one input to several outputs. While very cheap ones only copy the whole image, some models, often called wall controllers, are recognized by your system as a giant display, that is splitted into smaller outputs :
There are several cards on the market, but the Datapath FX4 is probably one of the best. It can split a big 4K HDMI/DisplayPort output into 4 smaller 1080p60 HDMI/SDI outputs. Please note that multiple FX4 can be synchronized together, see this article : How to synchronize multiple Datapath Fx4 ?
The AJA HA5-4K (4Kp60 HDMI into 4 x 1080 SDI) can also be a good choice, while the Magnimage MIG-F4 or the Bitvisus 104PRO are close to the FX4 (but it does not feature any synchronization system).
The Theatrixx DisplayPort to Quad SDI can receives 4 x 4Kp60 inputs then splits them into 16 x 3G-SDI 1080p60.
Some generic 4K wall controllers are sold around US$150 on various marketplaces, but be sure to check the resolution and frequency.
Years ago, the Matrox TripleHead was quite popular, but it tends not to work properly on recent computers. If you are using this card, be sure to check this compatibility sheat.
Video processors such as the Barco E2 can also do the same, but are of course more expensive.
Nowadays, you can also use 8K wall controllers such as the RGBLink D8, the Great Prosperity 8K Wall Controller or the Astro HDMI 2.1 to 2.0x4 converter. Indeed, recent computers such as the MacBookPro M2 Max feature a HDMI 2.1 output, so suitable for a 7680x4320@60 signal.
Blackmagic Cards
These are special device supported by Millumin, that can output to HDMI or SDI.
For example, the Ultrastudio Monitor 3G can output to 1080p60 HDMI/SDI. The Blackmagic Decklink 8K Pro is quite affordable and can manage up to 4 x 4K SDI outputs : with splitters, it means that you can manage 16 x 1080p60 outputs.
You can even embed such cards inside a Thunderbolt chassis, but keep in mind that Thunderbolt bandwidth will limit the outputs to 2 x 2160p60 (or 4 x 2160p30).
Lastly, it is interesting to note that the Decklink 8K Pro is able to output 4 x frame-sync 2160p SDI. To do, you need to use the device in 4320p mode by setting the connector to SDI 1 to 4 in or out in Blackmagic Desktop Video Setup. This way, it outputs to the whole device, the 4 SDI connectors at once (4320p in total, each SDI connector outputting to 2160p).
If needed, use a video-routing to easily manage your canvases to your outputs. See this article : Video Routing
DisplayLink
This is a technology that creates a virtual display on your computer, then sends its content to a compatible device. This requires to install drivers.
For example, the Startech USB to VGA Adapter connects via USB and can output 1920x1200. The Plugable USB-C Triple HDMI Display Docking Station connects via USB-C and supports 3 HDMI outputs.
However, from experience, frame drops can arise (this may be fixed in later DisplayLink update).
Docks
These are not splitters, it just provides more DisplayPort or HDMI ports via USB-C or Thunderbolt. Consequently, if your graphic card has a limitation in terms of numbers of outputs (such as for Apple Silicon computers), this will not help you to get more outputs.
Example of such docks is the CalDigit Mini Dock, that provides 2 HDMI outputs via Thunderbolt 3.
Other
On very different side, if your projector is suitable for NDI protocol, you can directly inject an output via a network cable. Or use a BirdDog PLAY to do so.
Also, eGPU can be a good alternative to get multiple outputs. However, they only works on with Intel computers, not with Apple Silicon ones.
More info in this article : Can Millumin use an eGPU ?
Lastly, you could use multiples machines all synchronized via a timecode.
See this article more more info : Timecodes