The number of elements varies with different religion. For example, in Wicca, there's earth, wind, fire, water and spirit. In Hinduism, there's earth, wind, fire, water, space (or void). In Zoroastrianism, there are two major elements: fire and water. There is no specific number for each religion represents their elements differently.
Darkness and Lightning are not Elements. Lightening, for instance, would fall under the Elements of Air and Fire; so it would not itself be an Element.
But another system which hasn't been mentioned are the Chinese Elements of Earth, Air, Fire, Wood and Metal.
While I don't recommend Wikipedia as always being a reliable source, their discussion of the various Elemental Systems is actually pretty good. You can find it at https://en.wikipedia.org/wiki/Classical_element
In my journey of knowledge these last few decades. I've come to learn that while they are known by different names when it comes to the various religions, they are in simplest of form as "Earth, Air, Fire, Water, and Soul" where as the Soul element is apart of everything from the tiniest grain of sand, to the vast reaches of the universe and beyond. Just like with all things there has to be balance as well. One element cant exist without the others. So choosing your elemental basis for things is important. You see this everywhere take example of without water Earth and fire would become destructive, without earth air and water would become destructive, and so forth. there has to be a balance within the elements or else chaos tends to ensue.