You can use one EV3 to control numerous other EV3's. This has the advantage of having just one platform. Another kind of platform is also possible, but then you'll have to writes lot of stuff yourself (communication, motor and sensor control).
I do think using 20 US sensors is a problem. The sensors will interfere, unless you fire them one by one. But then it will take at least a second, but probably more, to loop through them all.
My NXT blog: http://nxttime.wordpress.com/