5 required for both OM612 and OM647, five cylinder engines.
Existing glow plugs in your engine may be made by either Bosch or Beru. Both are original equipment suppliers to Mercedes. Beru is a wholly owned subsidiary of Borg Warner.
Sprinter 2002-2006 Do not try to remove glow plugs from a cold engine.
Before attempting to remove any glow plugs, start the engine and allow it to reach operating temperature. Failure to do so will probably result in shearing a glow plug off in the aluminum cylinder head. Before installing any new glow plug, coat the new glow plug threads with a good quality anti-seize compound, to facilitate easier removal in the future.