I need to know how can I estimate the amount of glass fiber in composite produced by poltrusion, based on the force that final product is supposed to withstand?
You need to establish the relationship between the volume fraction of fibres (vf) and the mechanical properties of the produced (fibre/matrix) composite by the rule of mixture. You will have the mechanical properties of the composite from vf=zero (basically the mechanical properties of the matrix) to vf=1 (basically mechanical properties of the fibre). Then the mechanical properties of your manufactured composite should fall between these two limits, and gives you an estimate of the fibres' volume fraction. Have a look at the attached paper, it should give you some idea.