Few people know the solution to this puzzle, what is the correct number?

Gedaan: This image contains hidden numbers, can you find them all?

Gedaan: Can you count all the blocks in this image? The result is surprising!

Gedaan: Only very good eyes can spot the number 6 among the 9s in 6 seconds!

Gedaan: The internet goes crazy: 97% of people can’t solve this riddle