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[I'm new to this too, so don't rely on anything I say, but I think I've read a little bit more than you have.]
Layers of what exactly? You've got a wide choice of types of carbon fibre cloths to choose from with different weights, so the number of layers will depend a lot on the type(s) you use. It's also hard to predict in advance how strong a part will be, so it's always best to make prototypes and test them to destruction. Wherever you cut the cloth, you substantially weaken the effective strength of the cut fibres when in tension for many inches away from the cut ends, so you may not get anything like the strengths stated on datasheets. Given that you're going to be sitting over this thing and it may break under you, potentially leaving lethal shards pointing upwards, you want to over-engineer it heavily, bringing its weight close to the weight of the metal you're replacing, and that means you should be aiming for a modest weight saving at best. Weigh the metal part and then try to use perhaps 90% of that weight in cloth+resin (assume equal weights for each of these). Whether a core material like Soric would help in a case like this, that I don't know, but someone who does know ought to comment on that - a fatter tube wall would be stronger and Soric would help get the resin through all the cloth, but it may not be possible to work it into such a tight, curved space and it may lead to broken outer layers when the post is clamped in the bicycle frame if the give is different under compression.
On a seat post, the back part will often be under compression while the front is in tension. I would think about using 45 degree biaxial 300gsm cloth aligned along the post, alternating it in layers so that you get half the fibres aligned up and down the post while a quarter are aligned 45 degrees to one side and the final quarter at 45 degrees to the other side. Put three or four layers of kevlar/cf cloth in between the outer layers of biaxial cloth (using the plain weave 200gsm) and the outermost layer should be twaron cloth to give the part a more attractive appearance (so from the outside inwards that's twaron, biaxial, kevlar, biaxial, kevlar, biaxial, kevlar, biaxial, biaxial, biaxial, biaxial...). The lower part of the post should perhaps be thicker as it will be clamped into a frame, but how much thicker is hard to know without testing to destruction.
Finding a viable construction method may be your biggest problem though. You'll probably need to pack all the cloth you use into a cylindrical mould with a tubular bag running through the middle which you can either inflate to a high pressure or use it to seal the cloth in against the mould so that you can infuse resin through it (the cloth under a vacuum). I can imagine a lot of failed attempts at this with the resin failing to infuse all the way through, so it may be an expensive business. I have searched for tips on this and found very few, and I still don't know how you can get all the cloth into the right place for the vacuum to be able to compress it outwards against the mould properly without it pulling tight everywhere and leaving voids where it can't reach the outside. Perhaps there is someone here who has done it and can explain how, but that's something you will certainly need to get answers on.
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