If using 6 to 8 limes instead of lime juice, put water in blender or food processor, set processor on puree, and with processor running drop lime slices in, one at a time. Key limes are very tough - do not drop in too many at one time. Drop in 8 to 10 slices, maybe more if you have a really powerful blender or processor. Process completely, then pour through strainer into another container. Make sure to press out all of the juice, discard lime remains then pour the lime juice back into blender. Again, drop in lime slices, repeating the above process until you have processed all of the limes. Set aside 1 cup of the resulting lime juice.
Preheat oven to 350 degrees F.
Blend key lime juice with the sweetened condensed milk.
Add the sour cream. In a separate container, beat the eggs, then add to the key lime mixture and beat for 1 minute.
Pour mixture into the graham cracker crust.
Bake for 10 minutes.
Remove from the oven and cool pie completely, about 1 hour. Top with whipped topping and put in freezer. Freeze at least 3 hours.
Serve straight from freezer.
Cutting the pie is made easier by wetting a knife in hot water before slicing.
