hi

does anyone have answer to how to define nr. of layers to obtain desired strength of object.

i would like to do seat post tube for my MTB.
so how to define what would be the minimum amount of layers that tube will still support my weight and not to have too much of not needed weight.

i know there is no 100% accurate way, but at least a point to start

I would disagree with building it to the same weight as the metal part it is replacing, though to some extent that does depend on the mechanical strength required. The general rule of thumb is to build it to the same THICKNESS as the metal part it is replacing. So if the original part is a 2mm thick steel tube, then a 2mm thick carbon tube should be ideal.