There are multiple ways to do multi-output. On a MacBook Pro for instance, there are two thunderbolt port where you can plug MiniDP to DVI / HDMI / VGA / Display port adapters. There is also an HDMI output. So you can already plug 3 external screens / projectors.
There are also splitters, we used a lot Matrox Dual / TripleHead2Go to go from one Mini DisplayPort to 2 or 3 DVI (you can then use passive DVI-HDMI adapters). However their software to setup is not very reliable. We always succeed setting it up, but it's time consuming. Maybe it's better supported on windows. But well, it's popular and unexpansive.
Better hardware: http://www.datapath.co.uk/multi-display ... atapath-x4
But more expansive...
On the computer side, it depends. On my MacBookPro late 2013, I could mix 4 HD movies on 4 full HD displays without frame drop. I think we can do more but I don't remember the limit. There can be a limit on the CPU (movie decompression) or on the GPU. I think movie decompression won't be a problem. On the GPU side, doing 4 full HD on a MacBook Pro might be limit, but if you're doing 720p you're more than ok.
On a Mac Pro 6 full HD outputs runs fluently.