There is no limit in Millumin about displays, the only limit is the hardware : in bref, how many displays you can physically connect to your computer.
It is recommanded to use active adapters to connect your displays, such as the Accell Mini DisplayPort to DVI-D. For a better understanding about active VS passive adpaters, you can read this interesting post.
If you want to add more outputs to your computer, you can use a multi-display adapter.
There are several cards on the market, but the Matrox TripleHead is probably the most famous one. If you are using this card, be sure to check this compatibility sheat.
Here are more devices :
- Datapath FX4 : 4K HDMI/DisplayPort into 4 x 1080p60 HDMI/SDI
- AJA HA5-4K : 4Kp60 HDMI into 4 x 1080 SDI
- Magnimage MIG-F4 : 4K HDMI/DisplayPort into 4 x 1080p60 HDMI/DVI
- CalDigit Mini Dock : Thunderbolt 3 into 2 x 4Kp60 HDMI
- Pluggable Dual Display : Thunderbolt 3 into 2 x 4Kp60 HDMI
- Blackmagic MiniMonitor : Thunderbolt 1 into 1080p60 HDMI/SDI
- Startech USB to VGA Adapter : USB port into one VGA
- Zotac Dual HDMI (hard to find) : Thunderbolt 1 into 2 x 1080p60 HDMI
If you are interested in synchronizing multiple Datapath Fx4, please read this post.
On very different side, if your projector is suitable for NDI protocol, you can directly inject an output via a network cable. Or use the BirdDog Mini to do so (be sure to upgrade your firmware so you get the BirdDog Link).
Also, eGPU is becoming a very strong alternative to get multiple outputs, on a powerful graphic card, and at low price. For example, you can have an eGPU able to output to 3 x 4Kp60 (meaning 12 x 1080p60) for around US$800. Such an eGPU can be used with any Mac having Thunderbolt 3 (even a MacMini).
More info on this post.
Lastly, to keep several machines in sync, you can use LTC or MTC timecodes : more info on this post.