For unity 2019.1, i can get input from 2 controllers and 2 trackers poco input at most, usually 2 controllers and 1 tracker poco input only.
For unity 2018.3, i only can get 2 devices input.
e.g. 2 x controllers OR 1 controller and 1 tracker poco OR 2 trackers poco
If i turn on the controller, the controller will take over the trackers input immediately, which means i can only use either 2 of controller or trackers input.
p.s. if i turn on 2 controllers, trackers are not able to get input but still can track the position.