How many rechargeable batteries do you need to by to fill the cartridge?
For the D800 Battery Pack you need one (1) battery. You can use (8) AA batteries with the included AA holder, or you can use either one (1) EN-EL15 battery (the same as is in the camera) or you can use (1) EN-EL 18 battery (the same as used by the D4). If you get the EN-EL18 battery you will have to buy the BL-5 battery cover and the MH-26 battery charger.
The question that has to be asked is why do you need the Battery Grip? If you absolutely need a lot of shots available, it may be worth it to pay more than $600 extra to get the EN-EL 18 battery, battery cover and charger. If you absolutely need that 6fps, and are not in a studio, you can use either (8) AA batteries with the included holder or an EN-EL 18 battery. If you have big hands (like me), just get the EN-EL 15 battery. If you will be in a studio setting, I would recommend just getting an EP-5B and EH-5 AC adapter set, and plug it into the wall. The AC adapter set also gives you 6fps.
The D800 gets 5fps (frames per second). The battery grip with the EN-EL 15 battery also gets 5fps. Per page 104 of the Nikon D800 manual, every other option gives you up to 6fps.