I've only tested 1 at a time on my Pi, but we're running five per USB bus (10 per machine) on the old Dell 755 testers. If a current model Pi has comparable horsepower to half an office PC from like 2010 you'd be fine. Given a Pi will run something like 12-14 Compac F, 3-4 R909 shouldn't be difficult.

By the way, 1.5TH is the baseline spec. The last batch off testers were running 1.6-1.7TH and still had headroom. Kano and I have both tested units at 2TH and up. Of course the top-end of your units will be dependent on the hardware behaving nicely but I bet you could get 3.6TH just from the R909s.