It really depends on what you're sifting. Usually, I only need to sift out
fines from material that has already been roughly graded. So I would typically only need a single sieve.
If you have soil components that are larger than what you want in your bonsai soil, you need a second size to screen that out, and if you are into shohin or making a top dressing you might have need for a very fine sieve.
As far as layering soil within a pot, that's something I have never done, but several of the texts, particularly the older ones, recommend it.
Regards,
Matt