I recently ordered some Biotex Flax and Jute fabrics from you for a little experiment that I am doing, but I just wanted to inquire about the fabrics stiffness. I have read that the fabrics are similar to fibreglass in terms of stiffness, is that so? 

I could not give you a sure answer on how many but for a first test I personaly would go for 10 layers, if you over-engeneered it you could go less... Keep in mind that you could use a core, making it stiffer with less layers!