Well I stopped at 3 prewashes, that was definitely enough! My WNNN's are super absorbant after being washed a few times now - worn after only one wash and was still dry on the outside, now the inserts in the pockets aren't even that wet!
Heya, coming in late (again..) but just wanted to say that I was looking at the Itti Bitti official website, and apparently they prewash all their nappies 3 times - they recommend an extra 3 prewashes as that brings the total up to 6 which is a good number of prewashes for bamboo. So no need to go mad on the prewashes for Ittis!
I can only bear to wash once before using a new nappy, if at all! Hemp is the only fabric I make sure to prewash, as it has natural oils that hinder absorbency until washed out. As the others have said, the coatings on fabrics (to make their production easier) will need washed out too, and after repeated washings and dryings the fibres will be fluffier and absorb more.