In days gone by, you could make a knitting spool from an empty thread spool and some finishing nails. Nowadays, most thread spools are made from plastic and not suitable for such purposes. In any case, doing more than four posts was awkward and the bore on a thread spool was often a bit tight.
The models presented here range from four to ten posts with varying diameters to accommodate taste. As my hands are well used to the shape of a spool it has been modeled in that form.
Just to round things out, a needle is included. Though it will do when printed in plastic, one made of aluminum will be more durable.
The spools were printed at 0.3mm layer height with 4 shells and 20% fill. The posts are sturdy enough for use, but won't survive any abuse. I doubt printing with 100% fill would improve the situation as the issue really is their length compared to diameter.
The needle was printed at 0.1mm layer height (to minimize catching on the yarn) with 100% fill. The difference between the two models is the 'a' version has a modeled in support for the head that is easy to snap off. Without the support it peeled during print, ruining it. The other end still peels, but is not particularly consequential. Printing directly on acrylic should eliminate the need for the support.
As expected it is somewhat flexible at the head and would easily snap if twisted. It certainly has nowhere near the durability of an aluminum needle.