It can be as complex as your computer can handle. There isn't any limitation on complexity in Ubox, the real limiting factor is the computer.
Still though, right now, 2,000 objects would lag most peoples Ubox severely, or even crash it. Even 1,000 is enough to lag things badly.
With the multicore capability enabled in US3, that number will be increased, but you'd need serious processing capability to handle 200,000 because it doesn't just have to name and display them, it has to simulate them, which Celestia doesn't.