Some people arrive at the number after considerable thought, some do not. Some overthink it, some don't think enough. In some cases, the optimum may actually be a different number than the current number. I would say if you are going to be off, better to err on the side of underestimation -- better to have fewer children than you can handle than to have more children than you can handle.
A number is a number is a number. Mine is zero. What's yours?