If I were a native, I would probably say: "The number of cars which use fossil fuels is far more than that of the ones using renewable energy" - 'that of the ones' instead of 'that' because you are referring to 'the number of the cars using ...' the number of X is far more than the number of Y; can be changed to: the number of X is far more than that of Y hope this helps! : )

The other answers are fine, but if you really want to be technically correct: The number of cars which use fossil fuels is far more than that of those using renewable energy. Also, it would be better to use "that" in place of "which," since it's a defining relative clause. Alternatively, you could reduce the clause to "The number of cars using...."

In my opinion, you would get a less clumsy sentence by avoiding using “number”: “There are far more cars using fossil fuels than cars using renewable energy.” Had I written the sentence using “number” I would have used “higher” or “greater” and not “more”: “The number of cars using fossil fuels is far higher/greater than that of those using renewable energy.”

Neither. :( Sorry. It is a good question. The number must be larger than a number. So you are right to suspect the first one is wrong. The number of cars which use fossil fuels is far more than that for using renewable energy. <-- I added a "for" The number of cars which use fossil fuels is far more than the number using renewable energy. Hope this helps. :)

The first sentence is correct Teresa