U need 2 and they are around £10 a piece the Hella part nr. is : 8ka 152 601-001 u can find them on the Hella website.
Also U need to fit the motor units from the standard lights, they click right on the DB-lights. In the fuse box there should be a light signal with no fuse in it, and U can see the 2 metal connection pins in the hole. U can see if it´s the right one because if not,your low beam will go out when the higm beam is on as the H4 bulbs on the standard head lights.
The low beam is HB3 a bulb, and the high beam is H7, get some blue vision they look the best!
good luck