help with my soon to be cook stove?

The local power that is (Hunny), had decided we need a rocket stove. Outside. Temporarily, this will be the simple stacked brick one. like one of these.

However, I ve been talking up Matt Walker's Tiny House Cook Stove for years. He thought we would just jump in the car and go get some fire bricks last week... He didn't give me ANY lead time. You know, locating it, maybe possession of the funds? Ha!

Then, in the above thread, Phillip posted a YouTube showing how to MAKE fire bricks!

But I have questions.

The sand for the clay sand morter, should this be sharp sand, or would sandbox sand work?

Does the firebrick making in the YouTube seem correct? How often does commercial bricks fail, as one of his has?

Here is a diagram of Matt's riserless core from donkey boards.

How many firebricks are in there?