Well there is something i learned from the school of hard knocks. :S
The number of axles do not mean much. I went to a manufacturer in Sikeston MO to get my 16 foot flat bed. Duuh, first thing they asked was what capacity axles you want. Ole Dave thought thought 3K axles were plenty that day.
Since then I have always run into issues with the trailer being too puny for the load.
So I think what you are gonna haul and what safety margin you want is the first step. Then there is the issue of brakes. I got them on both axles.