UPDATE: I think the most likely candidate for the type of cultivar this is would be Echeveria "Neon Breakers". Would like to hear your opinion if this would be the best guess. Thank you!

One of the hybrids? Maybe E. 'Blue Frills'

If many leaves are yellowing, there is probably a watering problem - usually overwatering. It could be from underwatering too, but first happens more often. Insufficient light will cause color change, but not so much yellow. I am assuming they are indoors - could you keep any outside? If not, what kind of sunlight do you have? I would suggest using lots of perlite or pumice mixed with soil. Whatever I can see in photos, looks like very peaty mix (or cocopeat?) PPL using soil mix it with at least 50% of sifted perlite. Or you can use gritty mix - made of mostly inorganic substrates (grit, perlite/pumice, turface, scoria...)

#18 - Echeveria rundelli is a synonym for Echeveria setosa var. deminuta. Usually it is quite hairy/fuzzy. Few guesses: #14 - perhaps E. chroma - edit: likely 'BlackPrince' as Nik suggested #19 - looks like E. Perle von Nurnberg or E. gibbiflora #20 - maybe E. neon breakers or E. Blue curls or E. shaviana - not ure which one as they are quite similar looking; plant may be too small to tell and it may need more light to color