How many screws per joint?

Across a 13"-wide joint, I did 5. I decided to see what it was like after construction, because I could always add them.

I decided that I didn't need to bother. And then I nailed a 1/4" luan backing on--that this this is a LOT of bracing. But it was sturdy enough, I felt.

So I left them off.

5 across the width of the board provides goo wracking resistance in that direction, and a backing protects the assembly in the other directions.

The backing is how the inexpensive knock down bookcases get a lot of their wracking resistance.

It does not take that much of a back panel as long as it has enough fasteners spread over all the edges.

It is barely stiff cardboard, but when nailed every 6 inches in the perimeter it adds a lot of stiffness to the assembly.

Oh, good! My backing is glued and nailed every 4 inches, so that's great to hear!

And the pocket-hole joints are also glued.

Thanks for the help!