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.
You can also use a Zotac Dual HDMI (hard to find nowadays) or a Datapath X4 (more expensive but you can get 4 x 1080p30 outputs, even 4 x 1080p60 in the case of Datapath FX4).
On the other side, there are some cheap USB cards, such as the Startech USB to VGA Adapter.
The Blackmagic MiniMonitor is also a good solution, but may imply a few frames of latency.
On very different side, if you 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)
Lastly, to keep several machines in sync, you can use LTC or MTC timecodes : more info on this post.