As others have mentioned there are currently four lounges, one of them a temporary one - this is because one of the three permanent lounges is being renovated and is down to half its normal size.
Only two of the lounges are in the international area - you have to go through passport control for the other two (including the temporary one). If your onward flight is to a Schengen country (=most of the EU or Norway) you have to do that anyway, but otherwise you would normally be staying in the international area.
The reason there are two lounges is that there used to be a separate one (with showers, and a better-stocked bar) for top tier elites. The lounge near D/E is the one with showers, but you need to have a KLM (or is it KLM or NW) Platinum Elite card to use them. For ordinary mortals there is a free shower (bring your own towel!) across the corridor.
The E/F lounge still seems to be the more popular (even though they're now the same), but it's also the one that fills up first - you will sometimes see "Lounge full - please use the other one" signs outside the door.
Both lounges can get pretty full - if you want to rest you may even be better off in one of the public areas upstairs where they have comfortable reclining seats.