Math Challenge 19

Math does not involve variables that we can see only.

There are the logical deduction type whereby you have to visualise and come out with an answer.

Below is one good example that I would like it to be a challenge.
(Don't be frightened by this, it is just for fun.....)

Above you will find a stack of cubic boxes. There are the blue and the yellow cubes.

Question:
What is the least quantity of blue boxes must we use in order to hold the yellow boxes in the same place?

You may present your answer and how you arrive at the answer in the comment section.

Thanks for trying. And I am waiting for that interesting mathematical deduction...

Maths Is Interesting!

Cheers :-D