Calculate voltage of Led driver required for a given amount of led's
Somewhere between 9V and 12V the LED will take a current of 900mA. If it does this at 9V and you apply 12V you might be getting a forward current of 3 amps thru the LED. There is no data sheet for the device (that I can find) so it's impossible to be exact.

But, realistically you probably need to put in a current limiting resistor for each LED. What makes you think it has a constant current circuit built into it?

The ebay link is really-really crappy at giving details - for example it states that the output power is 10 watts - this of course is rubbish!