How many devices can be connected to the HD6's via Bluetooth at one time?

Though the HD6's are capable of 'remembering' up to 6 devices, you can only have 1 source device (phone, laptop, computer, etc) actively paired via Bluetooth with the HD6's at a time.

The HD6 Bluetooth LED light will blink and be discoverable for 5 minutes once powered on. If in that 5 minute window the speakers are paired to a Bluetooth source, the light will go solid. After the 5 minutes is up, the light will turn off and it will no longer be discoverable and able to pair to new sources, but it will still be on. If the LED is off, a previously connected device can still pair to it at any time from within their device's Bluetooth menu.

If you want to pair to to a new device, just tap the LED light once more and it will blink and be discoverable for another 5 minutes. If you're paired to something and want to disconnect from the HD6's, you can hold the button down the Bluetooth LED for a few seconds before it disconnects.