How many different levels of r g b and a can unity create in a Color?

For the Color struct, each channel is stored as a float (i.e. 32-bit floating point value) adding to a total size of 128 bits - 4,294,967,296 unique values per-channel that can exist outside the normalised range of [0,1].

How many different levels of r, g, b or a can unity display when using Color?

Number of possible color variants doesn't depend on input data structures alone but on your whole graphics pipeline as well (this is true for every engine).

Imagine pushing this fat 128bpp byte4x4 ( Color) struct through a transformation pipeline (both on CPU and GPU sides) producing some variant of 32bpp or 24bpp struct at the other end and not knowing about it - you may think you have RGBA{ float r,g,b,a; } but, in reality, it's more like RGBA{ byte r,g,b; } when it hits your screen.

Also if you setup your pipeline correctly so you have 4 floats landing on the GPU memory, and have to be very pedantic about it, remember that even some lighting and basic shader operations, mere rounding errors etc (not to mention post-processing) running on your GPU can limit final color palette as well.

(not to mention physical display device that will mess all that up)