I don't know why you would think there is only 1 in existance. Check out
http://en.wikipedia.org/wiki/Ferrari_FXX The picture there is a red FXX. Right there shows there are at least 2, and I'm willing to bet there are more than that.
edit: actually, by reading wiki, it says this
Ferrari has built 31 of these cars, and they have all been sold to preselected past Ferrari customers. 30 of the Ferraris are red but one, which was not sold and is kept by Ferrari, is black. I"m assuming you meant that there is only 1 BLACK FXX in existance. In that case...you are correct sir.