Traditionally stems are built up or steam bent from solid hardwood which holds fasteners well but I assume you're gluing. Softwoods generally glue well, pine, cedar, spruce are fine. Min bending radius varies with species, moisture content and grain; air-dried WRC doesn't like bending much.

Best so far: I teased 1/8" clear pine to a 6.5" radius after being dipped in water and left to dry off for 1/2 hour. Pretty good for kiln-dried. I made 26 3-ply ribs for a canoe with not a single break - incredible luck I suspect. I used Titebond III which tolerates moist wood but if using epoxy keep the wood dry and let it rest on the mold overnight without glue so it doesn't break during gluing. Experiment with your stock to establish the thickest lamination that will make the bend without cracking.

It depends what you have, air dried will bend more than kiln-dried, green the most but then you have to wait for months for full strength and epoxy may not hold well on it. Many kiln-dried woods can't manage what I did to the pine even when moistened and steaming is less effective on softwoods than hardwoods.

As for final thickness, it's your design, but my minimum thickness guideline is 3x ply thickness for joints where I can guarantee perfect fits and x4 or x5 where there is likely to be some gaps. This is based on make and break tests I did some years ago. For example, my stripper canoe has 4 mm planks over 3/4" (19 mm) thick built-up spruce stems. Outer stem is laminated from 1/16" hardwood and epoxied since it's exposed to water unlike the titebonded inner stem.