What the heck is a 'layer'? Never understood it.
As you know, on an acoustic piano, when you press a key with different force you'll get a different sound timbre, not just a different volume of sound. For example, the 'pianissimo' sound timbre is not just a 'fortissimo' sound timbre with lower volume. It's a totally different type of sound (usually much rounder and warmer)... And from pianissimo to fortissimo, the palette of subtle tonal changes could be very large, depending on the type and quality of the acoustic instrument.
Now, on an acoustic piano there is no concept of layers. If you keep pressing the same key you'll get a slightly different sound timbre at each repetition, because the hammer hits an already vibrating string, therefore adding new vibrations to old vibrations, thus generating a different sound timbre.
In the digital world, we can use up to 127 values (because of MIDI limitations) to map the force/velocity of a "key press" into a sound. In a sample-based piano engine, there are samples for these mappings (not necessarily for all of them) and these samples are called "velocity layers". More velocity layers per key => sound timbre more faithful to the original in all the dynamic ppp->fff range.
Currently, what happens in most DP internal piano engines is that the manufacturer want to save costs as much as possible, so they store just a few velocity layers per key (usually 3-4 per key in middle-range models, 5-6 in top-range ones). The remaining layers usually are emulated with clever filtering and interpolation techniques.
In the VST world, ~10 layers per key is considered the bare minimum to get a decent palette of tonal changes. Some examples: the "Galaxy Vintage D" VST has 13 velocity layers per key. The "Ivory II American Concert D" VST has 20 layers per key. They also use techniques to emulate the missing layers, so that you should not hear a sudden transition from a layer to the next one, if you play a smooth crescendo or decrescendo.
Pianoteq generates algorithmically all the layers, so theoretically it's like you have the max amount of layers you can get in the limits of the standard MIDI specifications.