I keep seeing this pop up on my FB feed. To me, I only see 4. But the answers in the comments are all over the place. So what am I missing here?
I think it’s 18.
Taking just the area from the top parallel line upwards, there’s a left triangle, a middle triangle, a right triangle, a double size left (ie taking the leftmost vertical-ish line as the left of it, and the line two across from that to be the right of it), a double-size right, and a triple-size (whole top row).
That’s six already. Repeat again using the other two horizontal lines as the base in turn
I see 12. (hopeful!)
Four between the top point and the first horizontal line.
Another four between the top point and the second horizontal line.
Another four between the top point and the bottom line.
I count 18. The top 3rd of the drawing contains 6. Three individual ones, one made out of the first two segments, another made out of the second two segments, and another made out of all 3. Extend down another 3rd and you get 6 more doing the same thing. Another 6 with the last 3rd.
Dang!
What Aspidistra said. 
I’m really curious how you’re seeing four, because any count that includes four would include others akin to those four.
where’s our ninja emoji??
21 before I quit counting.
Hint: triangles that appear to overlap other triangles are still triangles.
I also got 18.
ETA: I finally read the article which confirms that it’s 18.
Yeah, I was counting the three obvious ones on top, then the one big one. I’m looking at it differently now after reading the previous comments in this thread.
18 for me. Curious about the 3 that got gnoitall to 21.
18 for me
I object to the word “guess” in the headline.
19, because the overall outline is also a triangle.
You’re counting it twice.
I think @Grrr’s answer is correct, because he is the only one who followed the instruction to guess how many triangles. All the rest of you are counting.
LOL. 
Took me a few tries, but I get 18 also.
I get 21. Consider the top horizontal “bar.” You can count three individual ones above it. You can combine the left and center…the center and the right…and all three combined. So I get six on that level.
Drop two the next horizontal…by extension you have six more. And at the bottom, six more. That’s 18 total.
But look at the center. You can make two more there. And one for the overall. That’s 21.
The answer is 24.