well I have the nimbus in front of me right now and it has 4 led's that denote both player number and % of charging. So I'm assuming you can use up to 4 controllers at the same time.
Actually, from the 'hands on with nimbus' article:
One would guess that the four-bar red light indicator on Nimbus’s face was there to show how much power is remaining, but it’s labeled “1, 2, 3, 4,” for reasons SteelSeries’ manual doesn’t explain. The numbers don’t indicate which of four controller positions a given Nimbus is using; the new Apple TV supports only two game controllers at once, and even when two Bluetooth controllers were connected, both had only their first light on. According to the manual, this light will flash for 10 seconds when the power reaches the “20 minutes remaining” stage.
So I wonder what the '1,2,3,4' is for. Also how would you distinguish between player 1 and 2 if light #1 is the only one lit regardless of player number?