Understanding 'Av' in this equation and finding the number of poles in a DC motor
What is Av in that equation? Is it the gain of the differential amp?

\$A_V\$ is a common symbol for voltage gain. In this case it is the gain of the op-amp circuit (not the op-amp itself).
You get their equation by starting with
$$A_v I_{max} R_{sense} = V_{DD}-V_{Ref},$$
basically saying you want to choose the sense resistor so that when the motor current is at maximum, the output voltage will be at maximum.

If so, then based on their schematics on page 37 the gain would be 220. Putting these values in the equation you get Rsense to be 2.2mR but they've chosen a 400mR resistor. What am I missing?

I haven't read the entire 40-page document to try to track down the source of the discrepancy, but I suspect the schematic shows an initial guess at the sense resistor value, not tuned to the particular motor and drive conditions.

Also, how do I find the number of poles of the motor (pg 13)?

It's something you'd have to know about the construction of the motor. Possibly it is included in the motor's datasheet. Or you could call the manufacturer and ask them. Or you could measure it by mechanically driving the motor shaft at known RPM and measuring the ripple frequency generated on the electrical terminals.