Max SPI speed AD7767


I would like to know: what's the max SPI speed for the AD7767? 

I've been trying to acquire data from a load cell using the AD7767 along with an ESP32; the problem is that once every few batches of data a 0 occurs on the serial interface and I suspect the culprit might be the SPI speed. I've attached the code, the schematic and a printscreen of the serial interface down bellow, for a better perspective.
The load cell that I use is this one: 500Kg S Type Load Cell - RobotShop 
Also: I get really low values on the serial interface like 1000-1200 if I pull really hard on the load cell. Shouldn't I be getting a lot more considering the fact that full range of the ADC goes up to ~16.000.000? Should I be using two op-amps on load cell differential output in order to increase the gain?