One thing to consider on the newer design cars starting with the 203 chassis is that the keys have a "memory" feature, ie the key stores memories such as climate control preferences, seat and mirror positions and radio presets. HOWEVER this is only available on key #1 and #2. IF an ADDITIONAL key is ordered it will NOT have this memory function. If an original key goes bad it will need to be replaced with a "replacement" key to maintain this feature. The delaer needs to decide if the "bad" key is key #1 or #2.
To expand upon what DOC said about the number of keys:
The car comes with 2 master keys when new. You can order up to 6 more (8 total) and you can have up to that many (8 keys) at a time. Also each key can be replaced twice, so through the cars life you could have had 24 keys, but no more than 8 operative at any one time. If for some reason this is exceeded, several expensive parts would need to be replaced, including mainly the EIS (the thing the key goes in to) and the engine control module, which is mated (married MB calls it!) to the EIS.