How many Ender Crystals are in the End?

Without any Ender Crystals around, killing the Enderdragon is getting easier because they heal him.

But how many Ender Crystals are there in the PC Edition?

I did some rigorous testing (as otherwise known as spawning 20 worlds and then count the number of Ender Crystals in each).
Just make new worlds and use "/say @e[type=EnderCrystal]" in the end. Then, count the number of "entity.EnderCrystal.name" it produces.

My results were (in chronological order):
12, 16, 17, 14, 13, 15, 22, 8, 14, 8, 16, 5, 12, 9, 19, 16, 13, 24, 22 and 12.

So, my wild guess is around 4-24 Ender Crystals, based on how big the island is.

Basically, it's pure luck (or the seed you've used). The number of crystals you encounter in each world depends on the size of the End Island. I've noticed that the islands usually center around 12-20 crystals.

Because Ender Crystals (and their obsidian pillars) are generated randomly based on the island size (as @Shunwoo quoted from the wiki), there is NO definite number of Ender Crystals in any world. It will never have the same number of pillars (unless you use the same seed over and over*).

NB: *I do not know if seed affects the generation of obsidian pillars (apart from the number of chunks that the island consists of; the number of rerolls to hit the 20% chance that a pillar will generate), but I know it definately does affect island size.
So, for all we know, using the same seed over could result in worlds containing 18, 17, 18, 16, 19, 18 pillars respectively)

According to the Minecraft wiki:

Each chunk has a 20% chance of generating an obsidian pillar.

The number isn't fixed, it depends entirely on the size of the End, as it is based on the numbers of chunks your map of the End has. The bigger the island, the more pillars you'll have.

Edit : If you want a range of numbers of pillars, it's : [ 0; +∞ ] 0 when there is no island, and +∞ when you create an endless End map.