What guidelines exist for the number of screws needed to attach two pieces of wood?
I don't know if there's something for screws in wood, as wood's a rather strange material.

For bolts into steel, however, there's the AISC Manual (no prices listed, but expect it to be in the $300+ range); there used to be a separate book on joints, so you could calculate the strength based on the size of the fastener, bolt pattern, etc.  There was also a section for calculating the strength of different weld patterns.

Now, the rules for the optimal strengthy are going to be similar -- further apart will support a greater moment, so when the contact patch is a square, you want the first two to be in opposing corners.  If using four, fill all of the corners.  However, because it's wood, you have a chance of spliting, so you don't want to go too close to the edges.  (if you have to; pre-drill).

The other thing to remember is that with screws and bolts, the fasteners shouldn't be taking the full load -- they're pulling the structural material together, so that the load's transfered as friction ... this means if you see a gap between the two pieces, you need more fasteners.  If you have a really large contact patch, drop another screw in the middle.