Personally, I find that the single-layer Yamaha P-95 plays more musically than the 4-layer Casio PX-130. Some people feel differently. The GEM RP-X is a nice sounding module that uses a hybrid sampling + modeling approach, it has only one sampled layer, and I think most people agree that it sounds better than either of the other two I mentioned.

I find that the single-layer Yamaha P-95 plays more musically than the 4-layer Casio PX-130

...how many velocity levels and sample levels players could distinguish in the digital pianos they have played, and how many they thought in practice were necessary...

Assertions to trust my ear about keyboards I have never heard are not very useful in directing me towards a model...!

However, in answer to your question: how many sample layers do you need for a high quality simulation of a piano? My answer is 1, because volume, as has been pointed out above, is but one of the variables that are altered at each of the (theoretically virtually unlimited) steps that can be recognised by a DP, even if it only has one sample layer per note.

..... What I would miss with more limited samples would be the sympathetic resonance.

If I'm not wrong, the P85 is single layered, while the P95 has four layers.

the P95 also has only one layer. In fact, dewster has pretty convincing evidence that it's the exact same layer that is found in the P85. I've heard people say it has multiple layers in several places now. Not sure where that got started

Perhaps I could go some way to averting certain dear friends from wasting their time. I did invite experiences of digital pianos along with their specifications, whilst wondering aloud how limited a digital piano could be whilst still being satisfying on its own. It was a pity this thread has attracted mainly those who can say nothing about these pianos but instead try to enforce dogmas about 'buying with ears', on pianos I never heard of. So let me take some time out to soothe these people with "You're oh so right", and then hopefully now they can leave it and let the rest of us chew the cud about digital pianos we have heard...!

I understand wanting some guidance about what pianos to try to find and play, before locating a store. But the question "how many layers should I look for" is almost like asking "how many knobs and buttons should it have." There's no right answer, and by itself, the parameter doesn't mean a lot.

A more useful query, IMO, would be something like "My needs are xxxxx, and I'm looking for something in the price range of up to about $yyy. What are some models I should look like?" Needs would be things like how portable it should be, the importance of any non-piano sounds or functions, whether it should have speakers, etc. Ideally you could also mention some models you've played in the past for reference, i.e. things you have liked the sound or feel of, or ones that you have not liked the sound or feel of.

If you're curious about technical specs, i.e. how many velocity layers a piano has, how well blended they are, how many notes are sampled vs. stretched, how long the decays are, how noticeable the loops are, whether there are various kinds of string resonances, and so forth, you can read dewster's informative evaluations in the DPBSD thread. But there is no single element among those that will define a superior piano; in number of velocity layers, you've just chosen one of many possible attributes to consider. Moreover, while it is of academic interest and may guide someone to understanding why they like certain models over others and what other ones may offer similar attributes, the fact is that even those tests will not tell you which of those pianos you will like best, and you can easily end up preferring one that "specs out" worse than some other in some notable way.