Just starting to use footers myself. Started with my TT, that has 3.
From what I have been reading (lots of white papers, feel like I'm back in school) the number of footers doesn't matter much, or really what you use. What matters most is that what you use is in the weight spec of the device. So if the device is 10lbs and you are using 4 footers you want to be in the middle of the weight spec. so you would use footers that support 1-3 lbs for that 10 lbs device.
Think the main reason for using more or less footers is trying to find that middle weight spec, and/or cost. Buying 3 footers will save a lot of money if doing several pieces of equipment. But also, anything that is a little heavy, should have a solid base.