There's shells and then there's shells. If you've never carved before, I would recommend a simple shell something like the one in the picture (in mahogany). I've taught one day (6 hour) courses in how to carve this shell and the students seem to be able to accomplish it pretty well - but that's with hands on instruction.
You'd need a #12/6, a #2/5, a #2/20, and a #8/4 to carve it (all numbers in the Pfeil system).
Mike