load cell number and accuracy
I you have 2 load cells then the supporting area is a line, which is hard to balance an object on. If you use a separate support then you aren't measuring what part of the object is supported by that support.

So you either need to hang the load or need at least 3 load cells to have a large enough area to make the object stable. 

Adding a 4th load cell is then done to make the usable area of your platform a rectangle which tends to be easier to work with.