A major limit on large multiseat linux configurations is going away. According to an article on the H Open website, "Kernel Log - Coming in 3.7 (Part 4): Drivers " by Thorsten Leemhuis, the 32 input device limit is gone as of Kernel 3.7. This means multiseat users may actually deploy as many Plugable seats as their system can drive. Now there is a tradeoff between the number of possible seats and the features enabled for each seat. The new device headroom may make work on multiseat development more likely, since it scales up better than before.
Thanks for posting about this Jeffrey! Yes, it’s great news that the evdev limit is gone in the latest kernels.
Note that the next limit is USB 2.0’s inherent limit of 127 devices/hubs per host controller. So you’ll still only be able to have 24 DC-125s http://plugable.com/products/dc-125/ at absolute maximum per host controller (with four USB 7 port USB 3.0 hubs to connect them).
But you can (and often do) have multiple USB host controllers per system. Intel’s recent chipsets have two separate USB host controllers, typically. But it’s hard to know which ports map to which.
But like you say, other “soft” scaling issues (CPU, memory, total pixel change rate across all screens) are probably the dominant issues anyway as you start getting to 8, 16, and more USB terminals per server.